diff --git a/CLAUDE.md b/CLAUDE.md
index 450312a..b2d58fc 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -6,7 +6,9 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 Two packages are merged to main and working:
 - `@gitmarks/core` (`packages/core/`) — schemas, GitHub Contents API client with optimistic concurrency, ULID/URL helpers (incl. opt-in tracking-param stripping), pure mutation helpers, example fixtures. 65 unit tests.
-- `@gitmarks/extension-chrome` (`packages/extension-chrome/`) — MV3 Chrome extension with toolbar save, two-way native-tree sync, 5-min poll, initial reconciliation, opt-in tracking-param stripping settings flag. 97 unit tests + 4 Playwright e2e tests (2 e2e skipped pending Playwright limitation fixes — see issue history).
+- `@gitmarks/extension-shared` (`packages/extension-shared/`) — canonical owner of the cross-browser extension code: popup, options, background, all of `src/lib/`, and the chrome/browser stub. 96 unit tests live here. Consumed by both browser shells via `workspace:*`. Uses `browser.*` via `webextension-polyfill`.
+- `@gitmarks/extension-chrome` (`packages/extension-chrome/`) — Chrome MV3 shell. Manifest + Vite/crxjs build + Playwright e2e (4 passing, 2 skipped — see issue history for the activeTab/Playwright limitation). Source files are thin entries that re-export from `extension-shared` via its `exports` map.
+- `@gitmarks/extension-firefox` (`packages/extension-firefox/`) — Firefox MV3 shell. Manifest + plain Vite build + manual smoke test (Playwright Firefox doesn't reliably drive WebExtensions). Targets Firefox 121+ for MV3 SW parity. Load via `about:debugging` → "Load Temporary Add-on".
 
 Pending packages (in dependency order): Firefox build, web UI (read + search + tags), web UI (write + bulk ops), Safari.
 
@@ -88,7 +90,7 @@ pnpm --filter @gitmarks/extension-chrome e2e
 1. ✅ `@gitmarks/core`
 2. ✅ Chrome MVP (toolbar save)
 3. ✅ Chrome native tree integration
-4. ⬜ Firefox build (`webextension-polyfill`) — issue [#23](https://github.com/paperhurts/gitmarks/issues/23)
+4. ✅ Firefox MV3 add-on (`webextension-polyfill` + extension-shared) — issue [#23](https://github.com/paperhurts/gitmarks/issues/23)
 5. ⬜ Web UI v1: list / search / tag management — issue [#24](https://github.com/paperhurts/gitmarks/issues/24)
 6. ⬜ Web UI v2: bulk operations + trash + export — issue [#25](https://github.com/paperhurts/gitmarks/issues/25)
 7. ⬜ Safari (`safari-web-extension-converter`) — issue [#26](https://github.com/paperhurts/gitmarks/issues/26)
diff --git a/README.md b/README.md
index 4c3a518..ce5ec5e 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,8 @@ you control.
 
 **Status:** Chrome extension is functional end-to-end (save via toolbar
 button, two-way sync with the native bookmark tree, 5-min poll for remote
-changes, automatic conflict retry). Firefox / Safari / web UI are next in
+changes, automatic conflict retry). Firefox MV3 add-on shipping the same
+source as Chrome via a shared package. Safari / web UI are next in
 the roadmap. See `spec.md` for the full design.
 
 ## Features (Chrome, today)
@@ -32,7 +33,9 @@ the roadmap. See `spec.md` for the full design.
 | Package | Role |
 |---|---|
 | `@gitmarks/core` | Shared TypeScript library: schemas (Zod), GitHub Contents API client with optimistic concurrency, ULID + URL helpers, pure mutation helpers |
-| `@gitmarks/extension-chrome` | Chrome MV3 extension. Save tabs, two-way sync with the native bookmark tree, 5-min poll, reconciliation |
+| `@gitmarks/extension-shared` | Cross-browser extension source — popup, options, background, lib/ helpers. Consumed by both browser shells via `workspace:*`. 96 unit tests live here. |
+| `@gitmarks/extension-chrome` | Chrome MV3 shell. Manifest + Vite/crxjs build + Playwright e2e. Thin entry files import from `extension-shared`. |
+| `@gitmarks/extension-firefox` | Firefox MV3 shell. Manifest + plain Vite build. Same source as Chrome via `extension-shared`. Load via `about:debugging`. |
 
 ## Quick start (Chrome extension)
 
@@ -88,7 +91,7 @@ The repo is a pnpm workspace monorepo. Each package has its own
 ## Architecture
 
 ```
-[Chrome ext] [Firefox ext (planned)] [Safari ext (planned)]    [Web UI (planned)]
+[Chrome ext] [Firefox ext] [Safari ext (planned)]    [Web UI (planned)]
        \             |                       /                       /
         \            |                      /                       /
          v           v                     v                       v
@@ -122,7 +125,7 @@ The load-bearing invariants:
 - ✅ Chrome MVP — toolbar-button save flow
 - ✅ Chrome native tree integration — listeners, reconcile, poll loop
 - ✅ Tracking-param stripping (opt-in)
-- ⬜ Firefox build ([#23](https://github.com/paperhurts/gitmarks/issues/23))
+- ✅ Firefox MV3 add-on ([#23](https://github.com/paperhurts/gitmarks/issues/23))
 - ⬜ Web UI v1: list + search + tag management ([#24](https://github.com/paperhurts/gitmarks/issues/24))
 - ⬜ Web UI v2: bulk operations + trash + export ([#25](https://github.com/paperhurts/gitmarks/issues/25))
 - ⬜ Safari ([#26](https://github.com/paperhurts/gitmarks/issues/26))
diff --git a/docs/superpowers/plans/2026-05-24-gitmarks-firefox-build.md b/docs/superpowers/plans/2026-05-24-gitmarks-firefox-build.md
new file mode 100644
index 0000000..b00e887
--- /dev/null
+++ b/docs/superpowers/plans/2026-05-24-gitmarks-firefox-build.md
@@ -0,0 +1,960 @@
+# Gitmarks Firefox Build Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Ship `@gitmarks/extension-firefox` — a Firefox MV3 add-on that does everything the Chrome extension does (popup save, two-way native-tree sync via `browser.bookmarks.*`, 5-minute poll, initial reconciliation, opt-in tracking-param stripping). Achieve this without code duplication by extracting the bulk of extension code into a new `@gitmarks/extension-shared` workspace package and turning `extension-chrome` + `extension-firefox` into thin browser-specific shells.
+
+**Architecture:** Three workspace packages where there were two — `@gitmarks/core` (unchanged), `@gitmarks/extension-shared` (new: all the cross-browser code), `@gitmarks/extension-chrome` (manifest + vite config + thin entries; imports from extension-shared), `@gitmarks/extension-firefox` (mirror of chrome's shell, Firefox-specific manifest). `webextension-polyfill` lets source code use `browser.*` everywhere; Chrome's `chrome.*` is auto-aliased.
+
+**Tech Stack:** TypeScript ESM throughout, `webextension-polyfill@^0.10`, Vite + `@crxjs/vite-plugin` for Chrome, Vite + manual manifest bundling for Firefox (crxjs is Chrome-only). Firefox 121+ for SW parity.
+
+**Spec reference:** `spec.md` §"Build order" — Firefox build (~½ day). Issue #23.
+
+**Out of scope (deferred to later plans / issues):**
+- AMO signing / distribution beyond developer-mode unpacked
+- Cross-browser Playwright e2e (unit tests cover shared logic; manual smoke verifies Firefox wiring)
+- Older Firefox event-page fallback for pre-121 — only target MV3 SW for now
+
+---
+
+## Decisions locked in upfront
+
+- **Polyfill at runtime, not at type level.** Source uses `browser.*` and imports the polyfill at the top of each entry (`background`, `popup`, `options`). TypeScript types from `webextension-polyfill` provide compile-time signatures. This avoids a build-time codemod and keeps both browsers' code paths identical.
+- **`extension-shared` is workspace-private.** `"private": true`, no published exports config — it's an internal monorepo package consumed via `"@gitmarks/extension-shared": "workspace:*"`. Tree-shaking lets the shells pull only what each entry needs.
+- **The shells own their manifests and Vite configs.** Each browser's `manifest.config.ts` + `vite.config.ts` live in its own package. The shared package owns no manifest — it's pure source.
+- **Tests live with the shared package**, not the shells. `extension-shared/test/` has the chrome-stub + all current unit tests. The shells have only e2e (Chrome) or manual smoke (Firefox).
+- **No behavior change in Chrome** as part of this plan. The refactor is a code move; the polyfill is a no-op against `chrome.*`. Every step keeps the existing 97 extension unit tests + 4 Playwright e2e green.
+- **Firefox extension ID** is set in the manifest via `browser_specific_settings.gecko.id`. Use `gitmarks@paperhurts.dev` (placeholder; per spec the project ships as developer-mode unpacked first).
+
+---
+
+## File structure (final)
+
+```
+packages/
+├── core/                                  # unchanged
+├── extension-shared/                      # NEW: all cross-browser code
+│   ├── package.json                       # name: @gitmarks/extension-shared
+│   ├── tsconfig.json
+│   ├── vitest.config.ts                   # moved from extension-chrome
+│   ├── src/
+│   │   ├── background.ts                  # moved
+│   │   ├── popup.html                     # moved
+│   │   ├── popup.ts                       # moved (browser.* via polyfill)
+│   │   ├── options.html                   # moved
+│   │   ├── options.ts                     # moved
+│   │   └── lib/                           # all moved
+│   │       ├── apply-remote.ts
+│   │       ├── background-core.ts
+│   │       ├── bookmark-factory.ts
+│   │       ├── bookmarks-file.ts
+│   │       ├── folder-path.ts
+│   │       ├── id-mapping.ts
+│   │       ├── listeners.ts
+│   │       ├── machine-id.ts
+│   │       ├── reconcile.ts
+│   │       ├── save-flow.ts
+│   │       ├── settings.ts
+│   │       └── suppression.ts
+│   └── test/                              # moved (chrome stub + 97 unit tests)
+├── extension-chrome/                      # thin shell
+│   ├── package.json                       # depends on extension-shared via workspace:*
+│   ├── manifest.config.ts                 # MV3 chrome manifest
+│   ├── vite.config.ts                     # crxjs plugin
+│   ├── playwright.config.ts               # unchanged
+│   ├── README.md                          # adjusted (manual smoke checklist points at shared)
+│   └── e2e/                               # Chrome-only e2e stays here
+│       ├── fixtures.ts
+│       ├── github-mock.ts
+│       ├── mvp.spec.ts
+│       └── sync.spec.ts
+└── extension-firefox/                     # NEW: thin Firefox shell
+    ├── package.json                       # depends on extension-shared via workspace:*
+    ├── manifest.json                      # MV3 Firefox manifest (JSON, no crxjs)
+    ├── vite.config.ts                     # plain Vite multi-entry build
+    ├── README.md                          # about:debugging dev workflow
+    └── src/
+        └── entries.ts                     # explicit import roots for each surface
+```
+
+---
+
+## Tasks
+
+### Task 0: Bootstrap `extension-shared` package skeleton (no migrations yet)
+
+**Files:**
+- Create: `packages/extension-shared/package.json`
+- Create: `packages/extension-shared/tsconfig.json`
+- Create: `packages/extension-shared/vitest.config.ts`
+- Create: `packages/extension-shared/src/.gitkeep` (placeholder so the directory exists in git)
+- Create: `packages/extension-shared/test/smoke.test.ts`
+
+**Why this task exists separately:** establishes the package shell and proves the toolchain works before we move 97 tests + ~2k LoC into it.
+
+- [ ] **Step 1: Create `packages/extension-shared/package.json`**
+
+```json
+{
+  "name": "@gitmarks/extension-shared",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "typecheck": "tsc -p tsconfig.json --noEmit",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "dependencies": {
+    "@gitmarks/core": "workspace:*",
+    "webextension-polyfill": "^0.12.0",
+    "zod": "^3.23.0"
+  },
+  "devDependencies": {
+    "@types/chrome": "^0.0.268",
+    "@types/webextension-polyfill": "^0.12.0",
+    "jsdom": "^25.0.0",
+    "vitest": "^2.0.0"
+  }
+}
+```
+
+- [ ] **Step 2: Create `packages/extension-shared/tsconfig.json`**
+
+```json
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "types": ["chrome", "vite/client"],
+    "rootDir": "./",
+    "outDir": "./dist-tsc",
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts", "test/**/*.ts", "vitest.config.ts"]
+}
+```
+
+(Note: we keep `@types/chrome` in `types` even though we're switching to `browser.*` — `@types/webextension-polyfill` re-exports the chrome types for compatibility, and the existing chrome stub in tests still references `chrome.bookmarks.BookmarkTreeNode` etc.)
+
+- [ ] **Step 3: Create `packages/extension-shared/vitest.config.ts`**
+
+```typescript
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "jsdom",
+    include: ["test/**/*.test.ts"],
+    setupFiles: ["./test/setup.ts"],
+    globals: false,
+  },
+});
+```
+
+(`setup.ts` doesn't exist yet — Task 1 moves it. For now, vitest will error on the missing setupFiles entry. The smoke test in Step 5 below works around that by deferring config use until Task 1.)
+
+- [ ] **Step 4: Create `packages/extension-shared/src/.gitkeep`**
+
+Empty file. Just `touch packages/extension-shared/src/.gitkeep`.
+
+- [ ] **Step 5: Create `packages/extension-shared/test/smoke.test.ts` and a stub `test/setup.ts`**
+
+Stub setup (will be replaced by the real one in Task 1):
+
+```typescript
+// packages/extension-shared/test/setup.ts (stub — replaced in Task 1)
+// Empty placeholder so vitest's setupFiles entry resolves.
+export {};
+```
+
+Smoke test:
+
+```typescript
+// packages/extension-shared/test/smoke.test.ts
+import { describe, it, expect } from "vitest";
+
+describe("@gitmarks/extension-shared smoke", () => {
+  it("loads without errors", () => {
+    expect(1 + 1).toBe(2);
+  });
+});
+```
+
+- [ ] **Step 6: Install + verify**
+
+Run: `pnpm install`
+
+Expected: pnpm fetches `webextension-polyfill`, `@types/webextension-polyfill`, and resolves `@gitmarks/extension-shared` as a workspace package.
+
+Run: `pnpm --filter @gitmarks/extension-shared test`
+Expected: 1 test passes.
+
+Run: `pnpm --filter @gitmarks/extension-shared typecheck`
+Expected: exit 0.
+
+- [ ] **Step 7: Commit**
+
+```bash
+git add packages/extension-shared pnpm-lock.yaml
+git commit -m "chore(extension-shared): bootstrap workspace package skeleton
+
+Pre-cursor to extracting the cross-browser extension code out of
+extension-chrome so that extension-firefox (issue #23) can consume the
+same source without duplication. This commit creates only the package
+shell + a smoke test; subsequent tasks move code into it."
+```
+
+---
+
+### Task 1: Move `chrome.*` test stub and unit tests to extension-shared
+
+**Files:**
+- Move: `packages/extension-chrome/test/setup.ts` → `packages/extension-shared/test/setup.ts`
+- Move: `packages/extension-chrome/test/*.test.ts` (12 files) → `packages/extension-shared/test/`
+
+**Why this task:** The 97 unit tests are package-agnostic — they test pure modules + the chrome stub. Moving them first means the rest of the refactor is verified by an already-passing suite living in its new home.
+
+**Critical:** the test files currently import via relative paths like `"../src/lib/settings.js"`. After the move, those paths must still resolve. Since `src/` is moving in Task 2, the imports need a temporary adjustment OR the tests + src move together.
+
+**Cleaner approach:** move test + src together in Task 1. Re-scope Task 1 to:
+
+- [ ] **Step 1: Move all source and tests in one atomic operation**
+
+Use `git mv` to preserve history:
+
+```bash
+git mv packages/extension-chrome/src/background.ts packages/extension-shared/src/
+git mv packages/extension-chrome/src/popup.html packages/extension-shared/src/
+git mv packages/extension-chrome/src/popup.ts packages/extension-shared/src/
+git mv packages/extension-chrome/src/options.html packages/extension-shared/src/
+git mv packages/extension-chrome/src/options.ts packages/extension-shared/src/
+git mv packages/extension-chrome/src/lib packages/extension-shared/src/lib
+git rm packages/extension-shared/src/.gitkeep
+
+git mv packages/extension-chrome/test/setup.ts packages/extension-shared/test/setup.ts
+# Overwrite the stub setup we created in Task 0
+git mv packages/extension-chrome/test/apply-remote.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/background-core.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/bookmark-factory.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/bookmarks-file.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/folder-path.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/id-mapping.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/listeners.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/machine-id.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/reconcile.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/save-flow.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/settings.test.ts packages/extension-shared/test/
+git mv packages/extension-chrome/test/suppression.test.ts packages/extension-shared/test/
+# smoke.test.ts in extension-chrome can be removed; extension-shared has its own
+git rm packages/extension-chrome/test/smoke.test.ts 2>/dev/null || true
+```
+
+Note the imports inside the moved files use relative paths like `"../src/lib/settings.js"` — those still resolve because the relative position (`test/` ↔ `src/lib/`) is unchanged inside extension-shared.
+
+- [ ] **Step 2: Make extension-shared export the public API**
+
+Create `packages/extension-shared/src/index.ts`:
+
+```typescript
+// Re-exports so the browser-specific shells can import everything they need
+// from one place. Keep this in alphabetical order; if you add a new public
+// surface in src/lib/, add it here.
+
+export { applyRemoteChanges } from "./lib/apply-remote.js";
+export * from "./lib/background-core.js";
+export { buildBookmark, type BuildBookmarkInput } from "./lib/bookmark-factory.js";
+export {
+  BOOKMARKS_PATH,
+  emptyBookmarksFile,
+  updateBookmarksOrBootstrap,
+} from "./lib/bookmarks-file.js";
+export {
+  BOOKMARKS_BAR_FOLDER,
+  OTHER_BOOKMARKS_FOLDER,
+  folderPathFromNode,
+  splitFolderPath,
+  type SplitPath,
+  type TreeNode,
+} from "./lib/folder-path.js";
+export {
+  IdMap,
+  asNodeId,
+  asUlid,
+  type NodeId,
+  type Ulid,
+} from "./lib/id-mapping.js";
+export {
+  flushPending,
+  registerListeners,
+  __resetForTest,
+  type ListenerDeps,
+} from "./lib/listeners.js";
+export { getMachineId } from "./lib/machine-id.js";
+export { reconcile } from "./lib/reconcile.js";
+export {
+  saveBookmark,
+  type PageInfo,
+  type SaveOptions,
+  type SaveResult,
+} from "./lib/save-flow.js";
+export {
+  SettingsCorruptError,
+  clearSettings,
+  loadSettings,
+  saveSettings,
+  settingsSchema,
+  type Settings,
+} from "./lib/settings.js";
+export {
+  clearSuppression,
+  isNodeSuppressed,
+  isSuppressed,
+  suppress,
+  suppressNode,
+} from "./lib/suppression.js";
+```
+
+- [ ] **Step 3: Update extension-chrome's `vitest.config.ts` and remove its now-empty test dir + setup**
+
+Since extension-chrome no longer has unit tests of its own (everything moved to extension-shared), delete its `vitest.config.ts` and the now-empty `test/` directory. Keep `e2e/` and `playwright.config.ts`.
+
+```bash
+git rm packages/extension-chrome/vitest.config.ts
+# test/ should be empty now after Step 1's moves; rmdir explicitly
+rmdir packages/extension-chrome/test 2>/dev/null || true
+```
+
+Edit `packages/extension-chrome/package.json` to remove the `test` and `test:watch` scripts and the vitest devDep — extension-chrome's test surface is now just e2e:
+
+```json
+{
+  "name": "@gitmarks/extension-chrome",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "vite build",
+    "dev": "vite build --watch --mode development",
+    "typecheck": "tsc -p tsconfig.json --noEmit",
+    "e2e": "playwright test",
+    "e2e:headed": "playwright test --headed",
+    "pretest:e2e": "vite build"
+  },
+  "dependencies": {
+    "@gitmarks/core": "workspace:*",
+    "@gitmarks/extension-shared": "workspace:*",
+    "zod": "^3.23.0"
+  },
+  "devDependencies": {
+    "@crxjs/vite-plugin": "^2.4.0",
+    "@playwright/test": "^1.48.0",
+    "@types/chrome": "^0.0.268",
+    "vite": "^5.4.0"
+  }
+}
+```
+
+(Note: we keep `zod` as a direct dep because the popup imports `SettingsCorruptError` from the shared package, and any Settings construction also depends on zod transitively. Easier to keep it visible than rely on transitive resolution. `jsdom` and `vitest` are now in extension-shared.)
+
+- [ ] **Step 4: Update extension-chrome's source entries to import from `@gitmarks/extension-shared`**
+
+The current `src/background.ts`, `src/popup.ts`, `src/options.ts`, and the `manifest.config.ts` references to them all still exist in extension-chrome temporarily — actually wait, they were moved to extension-shared in Step 1.
+
+**The shell needs its own thin entry files** that import from the shared package. Create:
+
+`packages/extension-chrome/src/background.ts`:
+```typescript
+// Chrome shell entry — re-exports the shared background module so the
+// MV3 manifest can point at this file and crxjs can package it cleanly.
+import "@gitmarks/extension-shared/dist/background.js";
+```
+
+Wait — that's wrong. `extension-shared` has no build step (we set `noEmit: true` in its tsconfig). The shell needs to either:
+- (a) directly import the .ts source via TypeScript path resolution (works with crxjs because vite handles TS)
+- (b) emit a dist/ for extension-shared
+
+Option (a) is cleaner. Adjust extension-chrome's tsconfig.json to add path mapping:
+
+Wait — even simpler. The shell entry just imports the side-effecting modules. Since `@gitmarks/extension-shared` has `"type": "module"` and points its main at a file we control, we can have the shared package expose `./background.js`, `./popup.js`, `./options.js` as subpath exports of the SOURCE (no build).
+
+Actually let me reconsider. `vite build` (which extension-chrome uses) handles TypeScript natively when given a `.ts` entry. So extension-chrome's entry can simply re-export from extension-shared via TS imports:
+
+`packages/extension-chrome/src/background.ts`:
+```typescript
+import "@gitmarks/extension-shared/src/background";
+```
+
+But `@gitmarks/extension-shared/src/background` needs to be importable. Add an `exports` map to `extension-shared/package.json`:
+
+```json
+{
+  "name": "@gitmarks/extension-shared",
+  ...
+  "exports": {
+    "./src/background": "./src/background.ts",
+    "./src/popup": "./src/popup.ts",
+    "./src/options": "./src/options.ts",
+    ".": "./src/index.ts"
+  }
+}
+```
+
+Vite + TypeScript can resolve `.ts` from `exports` because vite handles TS transformation in-process.
+
+Actually that's brittle. Cleaner: have the shells own thin entries that just import:
+
+`packages/extension-chrome/src/background.ts`:
+```typescript
+// Shell entry — Chrome MV3 manifest points here. The actual implementation
+// lives in @gitmarks/extension-shared; this file's side-effects (registering
+// listeners, the alarm, the initial reconcile) come from importing it.
+import "@gitmarks/extension-shared/background";
+```
+
+And the package.json exports:
+```json
+{
+  "exports": {
+    ".": "./src/index.ts",
+    "./background": "./src/background.ts",
+    "./popup": "./src/popup.ts",
+    "./options": "./src/options.ts"
+  }
+}
+```
+
+- [ ] **Step 5: Update manifests + HTML to point at shell entries**
+
+`packages/extension-chrome/manifest.config.ts` already points at `src/popup.html`, `src/options.html`, `src/background.ts`. We need to keep the manifest pointing at chrome's local shell — so create:
+
+`packages/extension-chrome/src/popup.html`:
+```html
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks</title>
+  </head>
+  <body>
+    <main id="root">loading…</main>
+    <script type="module" src="./popup.ts"></script>
+  </body>
+</html>
+```
+
+`packages/extension-chrome/src/popup.ts`:
+```typescript
+import "@gitmarks/extension-shared/popup";
+```
+
+`packages/extension-chrome/src/options.html`: same pattern.
+
+`packages/extension-chrome/src/options.ts`:
+```typescript
+import "@gitmarks/extension-shared/options";
+```
+
+`packages/extension-chrome/src/background.ts`:
+```typescript
+import "@gitmarks/extension-shared/background";
+```
+
+(Note: the HTML files reference `./popup.ts` and `./options.ts` relative to the HTML — and those `.ts` files just re-export from the shared package. Vite handles the transitive TS resolution.)
+
+- [ ] **Step 6: Run the suites + verify zero behavior change**
+
+```bash
+pnpm install   # pnpm sees the new exports + workspace dep
+pnpm --filter @gitmarks/extension-shared test
+# Expect: 97 tests passing
+pnpm --filter @gitmarks/extension-shared typecheck
+# Expect: exit 0
+pnpm --filter @gitmarks/extension-chrome typecheck
+# Expect: exit 0
+pnpm --filter @gitmarks/extension-chrome build
+# Expect: clean build, dist/manifest.json + dist/src/popup.html + assets emitted
+pnpm --filter @gitmarks/extension-chrome e2e
+# Expect: 4 passing, 2 skipped
+```
+
+If any test fails: the move broke something. Investigate before committing.
+
+- [ ] **Step 7: Commit**
+
+```bash
+git add -A
+git commit -m "refactor: extract extension-shared workspace package
+
+Moves all of packages/extension-chrome/src (background, popup, options,
+lib/) and packages/extension-chrome/test (chrome stub + 97 unit tests)
+into a new @gitmarks/extension-shared workspace package. extension-chrome
+becomes a thin shell with its own manifest, vite config, and entries
+that re-export from extension-shared.
+
+Why: extension-firefox (issue #23) needs the same source. Without this
+refactor we would duplicate ~2k LoC across browsers.
+
+No behavior change. All 97 unit tests still pass; all 4 Playwright e2e
+tests still pass (2 documented skips remain). extension-chrome's
+dist/manifest.json is unchanged."
+```
+
+---
+
+### Task 2: Migrate `chrome.*` → `browser.*` via `webextension-polyfill`
+
+**Files:**
+- Modify: every `.ts` file in `packages/extension-shared/src/` and `packages/extension-shared/test/setup.ts`
+
+**Why:** Firefox exposes `browser.*` natively; Chrome doesn't. `webextension-polyfill` exposes `browser.*` in Chrome via a thin wrapper over `chrome.*`. By switching all code to `browser.*`, the same source runs in both browsers.
+
+**Strategy:** mechanical find-and-replace, BUT:
+- The chrome stub in `test/setup.ts` is stubbing `chrome.*` — it must be reworked to stub `browser.*` instead (or to stub both, since the polyfill bridges them in production).
+- The polyfill is async-by-default for callback-style APIs (it converts them to Promises). All our code already uses `await chrome.x.y(...)` so this is a no-op semantically.
+
+- [ ] **Step 1: Add polyfill import to each entry file**
+
+`packages/extension-shared/src/background.ts` (at top):
+```typescript
+import browser from "webextension-polyfill";
+```
+
+Same for `popup.ts`, `options.ts`. The import has side effects (registers the polyfill) AND exports the unified `browser` namespace.
+
+- [ ] **Step 2: Replace `chrome.` with `browser.` across `src/`**
+
+```bash
+# From the repo root:
+find packages/extension-shared/src -name '*.ts' -exec \
+  sed -i 's/\bchrome\./browser./g' {} +
+```
+
+Inspect the diff carefully. Some occurrences are in COMMENTS (`// chrome.storage.local`); those don't need to change semantically but it's fine to update them for consistency. Some are TYPE references (`chrome.bookmarks.BookmarkTreeNode`) — those need different handling because `@types/chrome` namespace types aren't auto-mirrored. Resolve type imports via `@types/webextension-polyfill`'s `Browser.Bookmarks.BookmarkTreeNode` etc., OR keep the type imports as `chrome.*` since `@types/webextension-polyfill` re-exports compatibility types.
+
+For pragmatism: keep `chrome.bookmarks.BookmarkTreeNode` etc. in type positions (they're still valid via @types/chrome), and only change the VALUE positions (`chrome.bookmarks.create(...)` → `browser.bookmarks.create(...)`). Refine the sed if it's too aggressive:
+
+```bash
+# More targeted: replace chrome. only when followed by a callable property
+# (bookmarks/storage/runtime/etc.) AND not preceded by an import/type keyword.
+# Easier: do the global replace, then revert the type-position regressions.
+```
+
+Use your judgment after running the find/replace. If types break, add `type Browser = typeof browser;` aliases or use the `@types/webextension-polyfill` type namespace.
+
+- [ ] **Step 3: Replace the chrome stub with a browser stub in `test/setup.ts`**
+
+The current stub registers a global `chrome` object. After the polyfill switch, source code reads `browser`. Update the stub to register both — `browser` for the new code paths and `chrome` for any straggler:
+
+```typescript
+// packages/extension-shared/test/setup.ts (excerpt)
+const stub = {
+  storage: { /* ... existing ... */ },
+  runtime: { /* ... */ },
+  bookmarks: { /* ... */ },
+  alarms: { /* ... */ },
+  tabs: { /* ... */ },
+};
+
+vi.stubGlobal("browser", stub);
+vi.stubGlobal("chrome", stub);   // safety: any straggler still works
+```
+
+The `webextension-polyfill` runtime checks `globalThis.browser` first; setting it directly short-circuits the polyfill in tests (which we want — we're testing logic, not the polyfill).
+
+- [ ] **Step 4: Verify**
+
+```bash
+pnpm --filter @gitmarks/extension-shared typecheck
+# Expect: exit 0
+pnpm --filter @gitmarks/extension-shared test
+# Expect: 97 tests pass
+pnpm --filter @gitmarks/extension-chrome build
+# Expect: clean build
+pnpm --filter @gitmarks/extension-chrome e2e
+# Expect: 4 passing, 2 skipped — same as before
+```
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add -A
+git commit -m "refactor(extension-shared): migrate chrome.* → browser.* via webextension-polyfill
+
+Cross-browser code now uses browser.* uniformly. The polyfill aliases
+chrome.* under browser.* in Chrome; Firefox exposes browser.* natively.
+
+Tests' chrome stub now also exposes itself as 'browser' for the same
+reason. No production behavior change in Chrome; this prepares the
+source for consumption by extension-firefox (issue #23)."
+```
+
+---
+
+### Task 3: Bootstrap `extension-firefox` package
+
+**Files:**
+- Create: `packages/extension-firefox/package.json`
+- Create: `packages/extension-firefox/tsconfig.json`
+- Create: `packages/extension-firefox/vite.config.ts`
+- Create: `packages/extension-firefox/manifest.json` (literal JSON, not a TS config — crxjs is Chrome-only)
+- Create: `packages/extension-firefox/src/background.ts`
+- Create: `packages/extension-firefox/src/popup.html`
+- Create: `packages/extension-firefox/src/popup.ts`
+- Create: `packages/extension-firefox/src/options.html`
+- Create: `packages/extension-firefox/src/options.ts`
+
+- [ ] **Step 1: Create `packages/extension-firefox/package.json`**
+
+```json
+{
+  "name": "@gitmarks/extension-firefox",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "vite build && node ./scripts/copy-manifest.mjs",
+    "typecheck": "tsc -p tsconfig.json --noEmit"
+  },
+  "dependencies": {
+    "@gitmarks/core": "workspace:*",
+    "@gitmarks/extension-shared": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/webextension-polyfill": "^0.12.0",
+    "vite": "^5.4.0"
+  }
+}
+```
+
+- [ ] **Step 2: Create `packages/extension-firefox/tsconfig.json`**
+
+```json
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "types": ["webextension-polyfill", "vite/client"],
+    "rootDir": "./",
+    "outDir": "./dist-tsc",
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts", "vite.config.ts", "scripts/**/*.mjs"]
+}
+```
+
+- [ ] **Step 3: Create `packages/extension-firefox/vite.config.ts`**
+
+```typescript
+import { defineConfig } from "vite";
+import { resolve } from "node:path";
+
+export default defineConfig({
+  build: {
+    outDir: "dist",
+    target: "esnext",
+    minify: false,
+    sourcemap: true,
+    rollupOptions: {
+      input: {
+        background: resolve(__dirname, "src/background.ts"),
+        popup: resolve(__dirname, "src/popup.html"),
+        options: resolve(__dirname, "src/options.html"),
+      },
+      output: {
+        entryFileNames: "[name].js",
+        chunkFileNames: "assets/[name]-[hash].js",
+        assetFileNames: "[name].[ext]",
+      },
+    },
+  },
+});
+```
+
+- [ ] **Step 4: Create `packages/extension-firefox/manifest.json`** (note: literal JSON, not TS — Firefox doesn't have crxjs's defineManifest)
+
+```json
+{
+  "manifest_version": 3,
+  "name": "gitmarks",
+  "version": "0.0.1",
+  "description": "Save bookmarks to your own GitHub repo.",
+  "permissions": ["storage", "activeTab", "bookmarks", "alarms"],
+  "host_permissions": ["https://api.github.com/*"],
+  "action": {
+    "default_popup": "popup.html",
+    "default_title": "gitmarks"
+  },
+  "options_ui": {
+    "page": "options.html",
+    "open_in_tab": true
+  },
+  "background": {
+    "service_worker": "background.js",
+    "type": "module"
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "gitmarks@paperhurts.dev",
+      "strict_min_version": "121.0"
+    }
+  }
+}
+```
+
+(Note: `options_page` in Chrome's manifest is `options_ui` in Firefox MV3 — same semantic, different field name.)
+
+- [ ] **Step 5: Create `packages/extension-firefox/scripts/copy-manifest.mjs`**
+
+Plain Vite doesn't bundle the manifest. We copy it into `dist/` post-build:
+
+```javascript
+// packages/extension-firefox/scripts/copy-manifest.mjs
+import { copyFileSync, mkdirSync } from "node:fs";
+import { resolve, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const root = resolve(here, "..");
+mkdirSync(resolve(root, "dist"), { recursive: true });
+copyFileSync(
+  resolve(root, "manifest.json"),
+  resolve(root, "dist/manifest.json"),
+);
+console.log("[firefox] copied manifest.json to dist/");
+```
+
+- [ ] **Step 6: Create shell entry files**
+
+`packages/extension-firefox/src/background.ts`:
+```typescript
+import "@gitmarks/extension-shared/background";
+```
+
+`packages/extension-firefox/src/popup.html`:
+```html
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks</title>
+  </head>
+  <body>
+    <main id="root">loading…</main>
+    <script type="module" src="./popup.ts"></script>
+  </body>
+</html>
+```
+
+`packages/extension-firefox/src/popup.ts`:
+```typescript
+import "@gitmarks/extension-shared/popup";
+```
+
+`packages/extension-firefox/src/options.html`:
+```html
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks — settings</title>
+  </head>
+  <body>
+    <main id="root">loading…</main>
+    <script type="module" src="./options.ts"></script>
+  </body>
+</html>
+```
+
+`packages/extension-firefox/src/options.ts`:
+```typescript
+import "@gitmarks/extension-shared/options";
+```
+
+- [ ] **Step 7: Install + verify build**
+
+```bash
+pnpm install
+pnpm --filter @gitmarks/extension-firefox typecheck
+# Expect: exit 0
+pnpm --filter @gitmarks/extension-firefox build
+# Expect: vite builds + copy-manifest script runs.
+# dist/ should contain: background.js, popup.html, options.html, manifest.json, assets/
+ls packages/extension-firefox/dist/
+# Expect: manifest.json + background.js + popup.html + options.html visible
+```
+
+- [ ] **Step 8: Commit**
+
+```bash
+git add -A
+git commit -m "feat(extension-firefox): bootstrap Firefox MV3 add-on package
+
+Plain Vite multi-entry build (crxjs is Chrome-only). Manifest is
+literal JSON copied into dist/ post-build via scripts/copy-manifest.mjs.
+
+Targets Firefox 121+ for MV3 service-worker parity. browser_specific_settings
+declares the gecko id and strict_min_version.
+
+Source files are minimal shells that re-export from @gitmarks/extension-shared
+— all the actual code (popup, options, background, lib/) is shared with
+extension-chrome via that workspace package.
+
+Closes #23."
+```
+
+---
+
+### Task 4: README + manual smoke test guide
+
+**Files:**
+- Create: `packages/extension-firefox/README.md`
+
+- [ ] **Step 1: Write the README**
+
+```markdown
+# @gitmarks/extension-firefox
+
+Firefox MV3 add-on. Save bookmarks to your own GitHub repo + two-way sync
+with the native bookmark tree. Functionally identical to the Chrome
+extension; both load the same source from `@gitmarks/extension-shared`.
+
+## Develop
+
+```bash
+pnpm --filter @gitmarks/extension-firefox build
+```
+
+Then in Firefox 121+:
+
+1. Go to `about:debugging` → "This Firefox".
+2. Click "Load Temporary Add-on…"
+3. Select `packages/extension-firefox/dist/manifest.json`.
+
+The extension loads as temporary — it'll be removed when you quit
+Firefox. For permanent installation you'd need to sign with AMO
+(deferred per `spec.md`).
+
+## First-run setup
+
+Same as the Chrome extension — see
+`packages/extension-chrome/README.md` "First-run setup". The popup,
+options page, and behavior are identical.
+
+## Manual smoke test
+
+The unit test suite (`pnpm --filter @gitmarks/extension-shared test`)
+covers all the shared logic that runs in both browsers. The Firefox-
+specific bits (manifest, build output, runtime behavior in Firefox's
+WebExtensions runtime) need a manual check:
+
+- [ ] Build, load via `about:debugging`, confirm the toolbar icon
+      appears and the popup opens.
+- [ ] Walk through the Chrome README's "Manual smoke test" sections
+      ("Popup + toolbar save" and "Native tree sync") in Firefox.
+      Everything should behave the same.
+- [ ] Check `about:debugging` → click "Inspect" on the gitmarks
+      add-on. The DevTools console should show the service worker
+      running with no errors. The 5-minute alarm should be visible
+      under Storage → Extension Storage.
+
+## Known limitations
+
+Same as the Chrome extension's "Known limitations" section in
+`packages/extension-chrome/README.md`. Notably:
+- Folder-delete cascade not handled (documented limitation; issue #2).
+- Cross-browser e2e isn't automated; Playwright's Firefox driver
+  doesn't fully support WebExtensions APIs. The shared unit tests
+  cover the algorithm; this manual smoke test covers the wiring.
+```
+
+(Note: replace `` ` `` ` (backtick-space-backtick) sequences in the prose above with real triple backticks.)
+
+- [ ] **Step 2: Update the root README to mention the Firefox package**
+
+In `README.md`'s "Packages" table, add a row:
+
+```markdown
+| `@gitmarks/extension-firefox` | Firefox MV3 add-on. Same functionality as Chrome via the shared package. Load via `about:debugging`. |
+```
+
+And in the roadmap, mark Firefox as done:
+
+```markdown
+- ✅ Firefox build (`webextension-polyfill`) ([#23](https://github.com/paperhurts/gitmarks/issues/23))
+```
+
+- [ ] **Step 3: Update CLAUDE.md package list**
+
+Add a bullet under "Project status":
+```markdown
+- `@gitmarks/extension-firefox` (`packages/extension-firefox/`) — Firefox MV3 shell over the shared package. Loads via `about:debugging` → "Load Temporary Add-on".
+```
+
+And note that `@gitmarks/extension-shared` is the canonical owner of the cross-browser code now.
+
+- [ ] **Step 4: Update `packages/extension-chrome/README.md` to reflect the refactor**
+
+Change:
+```markdown
+Chrome MV3 extension. Save bookmarks to your own GitHub repo, and keep
+Chrome's native bookmark tree in two-way sync with the JSON file.
+```
+
+To:
+```markdown
+Chrome MV3 extension shell. The bulk of the implementation lives in
+`@gitmarks/extension-shared`; this package owns only the Chrome-specific
+manifest, Vite + crxjs build configuration, and Playwright e2e tests.
+
+Functionally identical to `@gitmarks/extension-firefox` — both shells
+import the same source.
+```
+
+- [ ] **Step 5: Verify the full suite + commit**
+
+```bash
+pnpm test           # extension-shared 97/97 + core 65/65
+pnpm typecheck      # all packages clean
+pnpm build          # all packages emit dist/
+pnpm --filter @gitmarks/extension-chrome e2e   # 4 passing, 2 skipped
+```
+
+```bash
+git add -A
+git commit -m "docs(extension-firefox): README + cross-reference updates
+
+- packages/extension-firefox/README.md: load-via-about:debugging workflow,
+  pointer to the shared smoke-test checklist, known-limitations refs.
+- README.md, CLAUDE.md: add the firefox package to the packages list and
+  mark Firefox build done in the roadmap.
+- packages/extension-chrome/README.md: clarify it's now a thin shell."
+```
+
+---
+
+## Self-review summary
+
+**Spec coverage:**
+
+| Spec section | Covered by |
+|---|---|
+| spec.md §"Build order" — Firefox via webextension-polyfill | Tasks 0-3 |
+| Issue #23 scope — `extension-firefox` package consuming the same source | Tasks 0-3 |
+| webextension-polyfill shim for Chrome-vs-Firefox API differences | Task 2 |
+| Adapt popup/options pages | Implicit — they're vanilla HTML + browser.* already |
+| Cross-browser test infra | Deferred to follow-up; unit tests in extension-shared cover the shared logic |
+| Document dev workflow | Task 4 |
+
+**Out of scope explicitly (do not implement here):**
+
+- AMO signing / store distribution
+- Playwright e2e for Firefox
+- Firefox event-page fallback for pre-121
+- Refactoring the manifest configuration into a shared schema (each browser's manifest is small enough that duplication is fine)
+
+**Placeholder scan:** none.
+
+**Type/name consistency:** `@gitmarks/extension-shared` is the package name used uniformly across imports, package.json `dependencies`, and the workspace declarations. The `browser` import is uniform. The `chrome` global still works in tests via dual stubbing.
+
+**Verification:** by the end of Task 4, the repo has 3 workspace packages: `core` (unchanged, 65 tests), `extension-shared` (new, 97 tests + the chrome stub), `extension-chrome` (thin shell, 4 e2e passing + 2 skipped), `extension-firefox` (thin shell, builds cleanly). All typecheck + build pass. Firefox add-on manually verified via the README's smoke test (the agent executing this plan should run that smoke test if Firefox is available locally; otherwise note it as user-required).
diff --git a/packages/extension-chrome/README.md b/packages/extension-chrome/README.md
index 86269ae..3f9348f 100644
--- a/packages/extension-chrome/README.md
+++ b/packages/extension-chrome/README.md
@@ -1,7 +1,12 @@
 # @gitmarks/extension-chrome
 
-Chrome MV3 extension. Save bookmarks to your own GitHub repo, and keep
-Chrome's native bookmark tree in two-way sync with the JSON file.
+Chrome MV3 extension shell. The bulk of the implementation lives in
+`@gitmarks/extension-shared`; this package owns only the Chrome-specific
+manifest, Vite + `@crxjs/vite-plugin` build configuration, thin entry
+files, and the Playwright e2e suite.
+
+Functionally identical to `@gitmarks/extension-firefox` — both shells
+import the same source.
 
 ## Develop
 
diff --git a/packages/extension-chrome/package.json b/packages/extension-chrome/package.json
index 8d27dc5..c881e20 100644
--- a/packages/extension-chrome/package.json
+++ b/packages/extension-chrome/package.json
@@ -7,22 +7,19 @@
     "build": "vite build",
     "dev": "vite build --watch --mode development",
     "typecheck": "tsc -p tsconfig.json --noEmit",
-    "test": "vitest run",
-    "test:watch": "vitest",
     "e2e": "playwright test",
     "e2e:headed": "playwright test --headed",
     "pretest:e2e": "vite build"
   },
   "dependencies": {
     "@gitmarks/core": "workspace:*",
+    "@gitmarks/extension-shared": "workspace:*",
     "zod": "^3.23.0"
   },
   "devDependencies": {
     "@crxjs/vite-plugin": "^2.4.0",
     "@playwright/test": "^1.48.0",
     "@types/chrome": "^0.0.268",
-    "jsdom": "^25.0.0",
-    "vite": "^5.4.0",
-    "vitest": "^2.0.0"
+    "vite": "^5.4.0"
   }
 }
diff --git a/packages/extension-chrome/src/background.ts b/packages/extension-chrome/src/background.ts
index 6db3b93..367ed0f 100644
--- a/packages/extension-chrome/src/background.ts
+++ b/packages/extension-chrome/src/background.ts
@@ -1,120 +1,3 @@
-import { GitHubClient } from "@gitmarks/core";
-import { loadSettings, type Settings } from "./lib/settings.js";
-import { getMachineId } from "./lib/machine-id.js";
-import { IdMap } from "./lib/id-mapping.js";
-import { reconcile } from "./lib/reconcile.js";
-import { registerListeners } from "./lib/listeners.js";
-import { applyRemoteChanges } from "./lib/apply-remote.js";
-import { runMaybeReconcile, runPollRemoteOnce, toEtag } from "./lib/background-core.js";
-
-const RECONCILE_INTERVAL_MS = 60 * 60 * 1000;
-const POLL_ALARM_NAME = "gitmarks:poll";
-const RECONCILED_AT_KEY = "gitmarks:lastReconciledAt";
-const LAST_ETAG_KEY = "gitmarks:bookmarksEtag";
-
-let cachedBarId: string | null = null;
-let cachedOtherId: string | null = null;
-
-async function getBarOtherIds(): Promise<{ bar: string; other: string }> {
-  if (cachedBarId != null && cachedOtherId != null) {
-    return { bar: cachedBarId, other: cachedOtherId };
-  }
-  const tree = await chrome.bookmarks.getTree();
-  const root = tree[0];
-  if (root?.children == null) {
-    throw new Error("unexpected chrome.bookmarks tree shape");
-  }
-  let bar: string | null = null;
-  let other: string | null = null;
-  for (const child of root.children) {
-    if (child.id === "1") bar = child.id;
-    else if (child.id === "2") other = child.id;
-  }
-  if (bar == null || other == null) {
-    throw new Error("could not find Bookmarks Bar (id=1) or Other Bookmarks (id=2) in tree");
-  }
-  cachedBarId = bar;
-  cachedOtherId = other;
-  return { bar, other };
-}
-
-function buildClient(settings: Settings): GitHubClient {
-  return new GitHubClient({
-    owner: settings.owner,
-    repo: settings.repo,
-    token: settings.token,
-    branch: settings.branch,
-  });
-}
-
-async function maybeReconcile(): Promise<void> {
-  const settings = await loadSettings();
-  if (settings == null) return;
-
-  const stored = await chrome.storage.local.get(RECONCILED_AT_KEY);
-  const lastReconciledAt = typeof stored[RECONCILED_AT_KEY] === "number"
-    ? (stored[RECONCILED_AT_KEY] as number)
-    : 0;
-
-  await runMaybeReconcile({
-    now: Date.now(),
-    lastReconciledAt,
-    reconcileIntervalMs: RECONCILE_INTERVAL_MS,
-    runReconcile: async () => {
-      const { bar, other } = await getBarOtherIds();
-      const client = buildClient(settings);
-      const idMap = await IdMap.load();
-      const machineId = await getMachineId();
-      const nowIso = new Date().toISOString();
-      await reconcile(client, idMap, bar, other, machineId, nowIso, settings.stripTrackingParams);
-    },
-    setStorage: (items) => chrome.storage.local.set(items),
-    removeStorage: (key) => chrome.storage.local.remove(key),
-  });
-}
-
-async function pollRemoteOnce(): Promise<void> {
-  const settings = await loadSettings();
-  if (settings == null) return;
-  const client = buildClient(settings);
-  const stored = await chrome.storage.local.get(LAST_ETAG_KEY);
-  const rawEtag = stored[LAST_ETAG_KEY];
-  const etag = typeof rawEtag === "string" ? toEtag(rawEtag) : null;
-
-  await runPollRemoteOnce({
-    etag,
-    now: Date.now(),
-    client,
-    applyRemote: async (data) => {
-      const { bar, other } = await getBarOtherIds();
-      const idMap = await IdMap.load();
-      await applyRemoteChanges(data, idMap, bar, other);
-    },
-    setStorage: (items) => chrome.storage.local.set(items),
-    removeStorage: (key) => chrome.storage.local.remove(key),
-  });
-}
-
-registerListeners({
-  getClient: async () => {
-    const s = await loadSettings();
-    if (s == null) throw new Error("no settings");
-    return buildClient(s);
-  },
-  getIdMap: async () => IdMap.load(),
-  getBarOtherIds,
-  getMachineId,
-  getStripTrackingParams: async () => {
-    const s = await loadSettings();
-    return s?.stripTrackingParams ?? false;
-  },
-});
-
-chrome.alarms.create(POLL_ALARM_NAME, { periodInMinutes: 5 });
-chrome.alarms.onAlarm.addListener((alarm) => {
-  if (alarm.name === POLL_ALARM_NAME) {
-    void pollRemoteOnce();
-  }
-});
-
-void maybeReconcile();
+// Chrome shell entry — MV3 manifest points at this file. The real
+// implementation lives in @gitmarks/extension-shared.
+import "@gitmarks/extension-shared/background";
diff --git a/packages/extension-chrome/src/options.ts b/packages/extension-chrome/src/options.ts
index 3b2e656..a5aac97 100644
--- a/packages/extension-chrome/src/options.ts
+++ b/packages/extension-chrome/src/options.ts
@@ -1,117 +1 @@
-import {
-  GitHubClient,
-  GitHubAuthError,
-  GitHubError,
-  GitHubNotFoundError,
-} from "@gitmarks/core";
-import { loadSettings, saveSettings, SettingsCorruptError, type Settings } from "./lib/settings.js";
-
-const $ = <T extends HTMLElement>(id: string): T => {
-  const el = document.getElementById(id);
-  if (el == null) throw new Error(`#${id} not found`);
-  return el as T;
-};
-
-const tokenInput = $<HTMLInputElement>("token");
-const ownerInput = $<HTMLInputElement>("owner");
-const repoInput = $<HTMLInputElement>("repo");
-const branchInput = $<HTMLInputElement>("branch");
-const stripTrackingParamsInput = $<HTMLInputElement>("stripTrackingParams");
-const validateBtn = $<HTMLButtonElement>("validate");
-const saveBtn = $<HTMLButtonElement>("save");
-const status = $<HTMLParagraphElement>("status");
-
-function readForm(): Settings {
-  return {
-    token: tokenInput.value.trim(),
-    owner: ownerInput.value.trim(),
-    repo: repoInput.value.trim(),
-    branch: branchInput.value.trim() || "main",
-    stripTrackingParams: stripTrackingParamsInput.checked,
-  };
-}
-
-function setStatus(msg: string, kind: "ok" | "err" | "neutral"): void {
-  status.textContent = msg;
-  status.className = kind === "neutral" ? "" : kind;
-}
-
-async function loadIntoForm(): Promise<void> {
-  let s;
-  try {
-    s = await loadSettings();
-  } catch (err) {
-    if (err instanceof SettingsCorruptError) {
-      // Clear all form fields so the user can re-enter valid settings.
-      tokenInput.value = "";
-      ownerInput.value = "";
-      repoInput.value = "";
-      branchInput.value = "";
-      setStatus("Stored settings are corrupted — please reconfigure.", "err");
-      return;
-    }
-    throw err;
-  }
-  if (s == null) return;
-  tokenInput.value = s.token;
-  ownerInput.value = s.owner;
-  repoInput.value = s.repo;
-  branchInput.value = s.branch;
-  stripTrackingParamsInput.checked = s.stripTrackingParams;
-}
-
-validateBtn.addEventListener("click", async () => {
-  setStatus("validating…", "neutral");
-  let s: Settings;
-  try {
-    s = readForm();
-  } catch (err) {
-    setStatus(err instanceof Error ? err.message : String(err), "err");
-    return;
-  }
-  const client = new GitHubClient(s);
-  try {
-    await client.read("bookmarks.json");
-    setStatus("✓ valid PAT, repo exists, bookmarks.json found", "ok");
-  } catch (err) {
-    if (err instanceof GitHubNotFoundError) {
-      setStatus(
-        "✓ valid PAT, repo exists (bookmarks.json not yet created — will be on first save)",
-        "ok",
-      );
-      return;
-    }
-    console.error("[gitmarks] validate failed", err);
-    if (err instanceof GitHubAuthError) {
-      setStatus(
-        "PAT rejected — check the token is valid and has 'Contents: Read and write' scope on this repo.",
-        "err",
-      );
-      return;
-    }
-    if (err instanceof GitHubError && err.status >= 500) {
-      setStatus(
-        `GitHub is having issues (${err.status}). Try again in a minute.`,
-        "err",
-      );
-      return;
-    }
-    if (err instanceof Error && (err.message.includes("Failed to fetch") || err.message.includes("NetworkError"))) {
-      setStatus("Network error — check your connection and try again.", "err");
-      return;
-    }
-    setStatus(err instanceof Error ? err.message : String(err), "err");
-  }
-});
-
-saveBtn.addEventListener("click", async () => {
-  try {
-    await saveSettings(readForm());
-    setStatus("✓ saved", "ok");
-  } catch (err) {
-    console.error("[gitmarks] save settings failed", err);
-    setStatus(err instanceof Error ? err.message : String(err), "err");
-  }
-});
-
-void loadIntoForm();
+import "@gitmarks/extension-shared/options";
diff --git a/packages/extension-chrome/src/popup.ts b/packages/extension-chrome/src/popup.ts
index b3f6c84..c3afe82 100644
--- a/packages/extension-chrome/src/popup.ts
+++ b/packages/extension-chrome/src/popup.ts
@@ -1,134 +1 @@
-import { GitHubClient } from "@gitmarks/core";
-import { loadSettings, SettingsCorruptError } from "./lib/settings.js";
-import { getMachineId } from "./lib/machine-id.js";
-import { saveBookmark, type SaveResult } from "./lib/save-flow.js";
-import type { LastErrorRecord } from "./lib/background-core.js";
-
-const root = document.getElementById("root");
-if (root == null) throw new Error("#root not found");
-
-async function getActiveTab(): Promise<chrome.tabs.Tab | null> {
-  // When opened as a real extension popup, currentWindow refers to the
-  // browser window the user was in (not the popup's own floating window),
-  // so this returns the tab they were viewing. activeTab grants access to
-  // title + url for that one tab on user-gesture popup open; no broader
-  // tabs permission is required.
-  const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
-  if (tab != null && tab.url != null && !tab.url.startsWith("chrome-extension://")) {
-    return tab;
-  }
-  return null;
-}
-
-async function render(): Promise<void> {
-  let settings;
-  try {
-    settings = await loadSettings();
-  } catch (err) {
-    if (err instanceof SettingsCorruptError) {
-      root!.innerHTML = `<p class="err">Settings are corrupted — please reconfigure gitmarks.</p>
-        <button id="setup">Open settings</button>`;
-      document.getElementById("setup")?.addEventListener("click", () => {
-        chrome.runtime.openOptionsPage();
-        window.close();
-      });
-      return;
-    }
-    throw err;
-  }
-  if (settings == null) {
-    root!.innerHTML = `
-      <p class="title">Welcome to gitmarks.</p>
-      <button id="setup">Set up gitmarks</button>
-    `;
-    document.getElementById("setup")!.addEventListener("click", () => {
-      chrome.runtime.openOptionsPage();
-      window.close();
-    });
-    return;
-  }
-
-  const tab = await getActiveTab();
-  if (tab == null || tab.url == null) {
-    root!.innerHTML = `<p id="status" class="err">No active tab.</p>`;
-    return;
-  }
-
-  root!.innerHTML = `
-    <p class="title" title="${escapeAttr(tab.title ?? tab.url)}">${escapeText(tab.title ?? tab.url)}</p>
-    <button id="save">Save this page</button>
-    <p id="status"></p>
-  `;
-
-  const errStored = await chrome.storage.local.get("gitmarks:lastError");
-  const lastErr = errStored["gitmarks:lastError"] as LastErrorRecord | undefined;
-  if (lastErr != null) {
-    const banner = document.createElement("p");
-    banner.className = "err";
-    banner.style.fontSize = "0.8rem";
-    banner.style.marginTop = "0.5rem";
-    const label = lastErr.kind === "auth" ? "Background sync auth failed" : `Background ${lastErr.source} failed`;
-    banner.textContent = `${label}: ${lastErr.message}`;
-    root!.appendChild(banner);
-  }
-
-  const saveBtn = document.getElementById("save") as HTMLButtonElement;
-  const status = document.getElementById("status")!;
-
-  saveBtn.addEventListener("click", async () => {
-    saveBtn.disabled = true;
-    saveBtn.textContent = "saving…";
-    status.className = "";
-    status.textContent = "";
-    let result: SaveResult;
-    try {
-      const machineId = await getMachineId();
-      const client = new GitHubClient({
-        owner: settings.owner,
-        repo: settings.repo,
-        token: settings.token,
-        branch: settings.branch,
-      });
-      result = await saveBookmark(
-        client,
-        { url: tab.url!, title: tab.title ?? tab.url! },
-        machineId,
-        new Date().toISOString(),
-        { stripTrackingParams: settings.stripTrackingParams },
-      );
-    } catch (err) {
-      result = {
-        ok: false,
-        kind: "unknown",
-        message: err instanceof Error ? err.message : String(err),
-      };
-    }
-    if (result.ok) {
-      status.className = "ok";
-      status.textContent = "✓ saved";
-    } else {
-      status.className = "err";
-      status.textContent = result.message;
-      saveBtn.disabled = false;
-      saveBtn.textContent = "Try again";
-    }
-  });
-}
-
-function escapeText(s: string): string {
-  return s
-    .replace(/&/g, "&amp;")
-    .replace(/</g, "&lt;")
-    .replace(/>/g, "&gt;");
-}
-
-function escapeAttr(s: string): string {
-  return escapeText(s).replace(/"/g, "&quot;");
-}
-
-render().catch((err) => {
-  console.error("[gitmarks] popup render failed", err);
-  if (root != null) {
-    root.innerHTML = `<p class="err">Something went wrong opening gitmarks. Please reload the extension.</p>`;
-  }
-});
+import "@gitmarks/extension-shared/popup";
diff --git a/packages/extension-chrome/test/smoke.test.ts b/packages/extension-chrome/test/smoke.test.ts
deleted file mode 100644
index b4b1354..0000000
--- a/packages/extension-chrome/test/smoke.test.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-import { describe, it, expect } from "vitest";
-
-describe("@gitmarks/extension-chrome smoke", () => {
-  it("has chrome.storage stubbed by the global setup", () => {
-    expect(typeof chrome).toBe("object");
-    expect(typeof chrome.storage.local.get).toBe("function");
-  });
-});
diff --git a/packages/extension-chrome/vitest.config.ts b/packages/extension-chrome/vitest.config.ts
deleted file mode 100644
index 053baaf..0000000
--- a/packages/extension-chrome/vitest.config.ts
+++ /dev/null
@@ -1,10 +0,0 @@
-import { defineConfig } from "vitest/config";
-
-export default defineConfig({
-  test: {
-    environment: "jsdom",
-    include: ["test/**/*.test.ts"],
-    setupFiles: ["./test/setup.ts"],
-    globals: false,
-  },
-});
diff --git a/packages/extension-firefox/README.md b/packages/extension-firefox/README.md
new file mode 100644
index 0000000..8689e3f
--- /dev/null
+++ b/packages/extension-firefox/README.md
@@ -0,0 +1,121 @@
+# @gitmarks/extension-firefox
+
+Firefox MV3 add-on. Save bookmarks to your own GitHub repo + two-way
+sync with the native bookmark tree. Functionally identical to the Chrome
+extension — both load the same code from `@gitmarks/extension-shared`.
+
+## Develop
+
+```bash
+pnpm --filter @gitmarks/extension-firefox build
+```
+
+Then in Firefox 121+:
+
+1. Go to `about:debugging` → click **This Firefox**.
+2. Click **Load Temporary Add-on…**.
+3. Select `packages/extension-firefox/dist/manifest.json`.
+
+The add-on loads as temporary — it'll be removed when you quit Firefox.
+For permanent installation you'd need to sign with AMO (deferred per
+`spec.md` — ship as developer-mode unpacked first; do AMO review later
+if usage justifies it).
+
+## First-run setup
+
+Identical to Chrome — see `packages/extension-chrome/README.md`
+"First-run setup". The popup, options page, PAT validation, and the
+optional **Strip tracking parameters** toggle all behave the same.
+
+## Manual smoke test
+
+The unit test suite (`pnpm --filter @gitmarks/extension-shared test`,
+96 tests) covers all the shared logic that runs in both browsers. The
+Firefox-specific pieces (manifest, build output, runtime behavior in
+Firefox's WebExtensions runtime, native `browser.*` vs the polyfilled
+version Chrome uses) need a manual check:
+
+**Load + popup:**
+
+- [ ] Build, load via `about:debugging` → "Load Temporary Add-on", select
+      `dist/manifest.json`. The gitmarks toolbar icon appears as a default
+      puzzle piece (pin it for easy access).
+- [ ] Click the icon before configuring → popup shows "Set up gitmarks".
+
+**Setup flow:**
+
+- [ ] Click "Set up gitmarks" → options page opens in a new tab.
+- [ ] Enter PAT + owner + repo + branch + click Validate. Both green
+      success outcomes (file exists / file 404-but-repo-found) should
+      behave identically to the Chrome flow.
+- [ ] (Optional) Check "Strip tracking parameters" and Save.
+
+**Save flow:**
+
+- [ ] Navigate to any page, click the toolbar icon → "Save this page".
+      Green "✓ saved" within ~2s. Refresh `bookmarks.json` on
+      github.com — the new entry appears with `added_from: "chrome@<id>"`.
+      (The `chrome@` prefix is intentional — the polyfill exposes the
+      same `browser.*` namespace, but the machine-id helper writes the
+      prefix that the Chrome extension uses too. We may revisit this if
+      cross-browser disambiguation matters; for now, all `added_from`
+      values carry `chrome@` regardless of browser.)
+
+**Native tree sync:**
+
+- [ ] Drag any URL to the bookmarks toolbar. Within ~1 second the entry
+      appears in `bookmarks.json` on GitHub.
+- [ ] Right-click the bookmark in Firefox → Edit → change the title.
+      The remote `title` updates within ~1 second.
+- [ ] Delete the bookmark in Firefox. The remote entry gets a
+      `deleted_at` timestamp (soft delete).
+- [ ] Edit `bookmarks.json` directly on GitHub: add a new entry with a
+      fresh ULID, commit. Within 5 minutes the bookmark appears in
+      Firefox's bookmarks tree.
+
+**Trigger an immediate poll** (instead of waiting 5 min):
+
+1. `about:debugging` → click **Inspect** on the gitmarks add-on.
+2. Open the Console tab.
+3. Run: `browser.alarms.create("gitmarks:poll", { when: Date.now() + 1000 })`.
+
+## Known limitations
+
+Same as the Chrome extension — see the "Known limitations" section in
+`packages/extension-chrome/README.md`. Notably:
+
+- Folder-delete cascade not handled (issue #2; documented).
+- Cross-browser e2e isn't automated. Playwright's Firefox driver has
+  spotty WebExtensions support, especially for service workers; the
+  shared unit suite covers the algorithms and this manual smoke test
+  covers the wiring.
+
+## Architecture
+
+`@gitmarks/extension-firefox` is a thin shell. The full implementation
+lives in `@gitmarks/extension-shared`:
+
+```
+packages/
+├── core/                        # GitHub client, schemas, mutations
+├── extension-shared/            # cross-browser source (this is the brain)
+│   ├── src/background.ts        # SW: listeners + alarm + reconcile
+│   ├── src/popup.ts             # popup UI + popup-direct save flow
+│   ├── src/options.ts           # PAT/repo/branch + strip-tracking-params
+│   └── src/lib/                 # 12 pure-ish modules
+├── extension-chrome/            # Chrome shell (manifest + vite-crxjs + e2e)
+└── extension-firefox/           # this package: Firefox shell
+    ├── manifest.json            # MV3, gecko id, strict_min_version 121.0
+    ├── vite.config.ts           # plain Vite multi-entry (no crxjs)
+    ├── scripts/copy-manifest.mjs # copies manifest.json into dist/
+    └── src/
+        ├── background.ts        # → @gitmarks/extension-shared/background
+        ├── popup.ts             # → @gitmarks/extension-shared/popup
+        ├── popup.html
+        ├── options.ts           # → @gitmarks/extension-shared/options
+        └── options.html
+```
+
+`webextension-polyfill` lets the shared source use `browser.*`
+uniformly. Chrome's `chrome.*` is auto-aliased by the polyfill; Firefox
+exposes `browser.*` natively.
diff --git a/packages/extension-firefox/manifest.json b/packages/extension-firefox/manifest.json
new file mode 100644
index 0000000..7276473
--- /dev/null
+++ b/packages/extension-firefox/manifest.json
@@ -0,0 +1,26 @@
+{
+  "manifest_version": 3,
+  "name": "gitmarks",
+  "version": "0.0.1",
+  "description": "Save bookmarks to your own GitHub repo.",
+  "permissions": ["storage", "activeTab", "bookmarks", "alarms"],
+  "host_permissions": ["https://api.github.com/*"],
+  "action": {
+    "default_popup": "popup.html",
+    "default_title": "gitmarks"
+  },
+  "options_ui": {
+    "page": "options.html",
+    "open_in_tab": true
+  },
+  "background": {
+    "service_worker": "background.js",
+    "type": "module"
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "gitmarks@paperhurts.dev",
+      "strict_min_version": "121.0"
+    }
+  }
+}
diff --git a/packages/extension-firefox/package.json b/packages/extension-firefox/package.json
new file mode 100644
index 0000000..0334741
--- /dev/null
+++ b/packages/extension-firefox/package.json
@@ -0,0 +1,20 @@
+{
+  "name": "@gitmarks/extension-firefox",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "vite build && node ./scripts/copy-manifest.mjs",
+    "typecheck": "tsc -p tsconfig.json --noEmit"
+  },
+  "dependencies": {
+    "@gitmarks/core": "workspace:*",
+    "@gitmarks/extension-shared": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/chrome": "^0.0.268",
+    "@types/node": "^22.0.0",
+    "@types/webextension-polyfill": "^0.12.0",
+    "vite": "^5.4.0"
+  }
+}
diff --git a/packages/extension-firefox/scripts/copy-manifest.mjs b/packages/extension-firefox/scripts/copy-manifest.mjs
new file mode 100644
index 0000000..5107e9d
--- /dev/null
+++ b/packages/extension-firefox/scripts/copy-manifest.mjs
@@ -0,0 +1,12 @@
+import { copyFileSync, mkdirSync } from "node:fs";
+import { resolve, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const root = resolve(here, "..");
+mkdirSync(resolve(root, "dist"), { recursive: true });
+copyFileSync(
+  resolve(root, "manifest.json"),
+  resolve(root, "dist/manifest.json"),
+);
+console.log("[firefox] copied manifest.json to dist/");
diff --git a/packages/extension-firefox/src/background.ts b/packages/extension-firefox/src/background.ts
new file mode 100644
index 0000000..82fb1e3
--- /dev/null
+++ b/packages/extension-firefox/src/background.ts
@@ -0,0 +1 @@
+import "@gitmarks/extension-shared/background";
diff --git a/packages/extension-firefox/src/options.html b/packages/extension-firefox/src/options.html
new file mode 100644
index 0000000..16309e7
--- /dev/null
+++ b/packages/extension-firefox/src/options.html
@@ -0,0 +1,64 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks — settings</title>
+    <style>
+      body {
+        font-family: system-ui, sans-serif;
+        max-width: 480px;
+        margin: 2rem auto;
+        padding: 0 1rem;
+        color: #1a1a1a;
+        background: #fafafa;
+      }
+      h1 { font-size: 1.5rem; margin-bottom: 1.5rem; }
+      label { display: block; margin-bottom: 1rem; }
+      label > span { display: block; font-size: 0.85rem; margin-bottom: 0.25rem; }
+      input { width: 100%; padding: 0.5rem; font: inherit; box-sizing: border-box; border: 1px solid #ccc; border-radius: 4px; }
+      .row { display: flex; gap: 0.5rem; margin-top: 1.5rem; }
+      button { padding: 0.5rem 1rem; font: inherit; border: 1px solid #444; background: #222; color: white; border-radius: 4px; cursor: pointer; }
+      button.secondary { background: white; color: #222; }
+      #status { margin-top: 1rem; font-size: 0.9rem; min-height: 1.2em; }
+      #status.ok { color: #096; }
+      #status.err { color: #c00; }
+    </style>
+  </head>
+  <body>
+    <h1>gitmarks settings</h1>
+
+    <label>
+      <span>GitHub fine-grained personal access token</span>
+      <input id="token" type="password" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Repo owner (your GitHub login)</span>
+      <input id="owner" type="text" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Repo name</span>
+      <input id="repo" type="text" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Branch</span>
+      <input id="branch" type="text" autocomplete="off" spellcheck="false" value="main" />
+    </label>
+
+    <label class="checkbox">
+      <input id="stripTrackingParams" type="checkbox" />
+      Strip tracking parameters (utm_*, fbclid, gclid, msclkid, mc_*) at save time
+    </label>
+
+    <div class="row">
+      <button id="validate" class="secondary">Validate</button>
+      <button id="save">Save</button>
+    </div>
+
+    <p id="status"></p>
+
+    <script type="module" src="./options.ts"></script>
+  </body>
+</html>
diff --git a/packages/extension-firefox/src/options.ts b/packages/extension-firefox/src/options.ts
new file mode 100644
index 0000000..a5aac97
--- /dev/null
+++ b/packages/extension-firefox/src/options.ts
@@ -0,0 +1 @@
+import "@gitmarks/extension-shared/options";
diff --git a/packages/extension-firefox/src/popup.html b/packages/extension-firefox/src/popup.html
new file mode 100644
index 0000000..69a0b72
--- /dev/null
+++ b/packages/extension-firefox/src/popup.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks</title>
+    <style>
+      body {
+        font-family: system-ui, sans-serif;
+        margin: 0;
+        padding: 0;
+        background: #fafafa;
+        color: #1a1a1a;
+        min-width: 240px;
+      }
+      main { padding: 1rem; }
+      button {
+        width: 100%;
+        padding: 0.6rem;
+        font: inherit;
+        border: 1px solid #444;
+        background: #222;
+        color: white;
+        border-radius: 4px;
+        cursor: pointer;
+      }
+      button:disabled { opacity: 0.6; cursor: progress; }
+      #status { margin-top: 0.75rem; font-size: 0.85rem; min-height: 1.2em; }
+      #status.ok { color: #096; }
+      #status.err { color: #c00; }
+      .title { font-size: 0.85rem; color: #555; margin: 0 0 0.5rem 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+    </style>
+  </head>
+  <body>
+    <main id="root">loading…</main>
+    <script type="module" src="./popup.ts"></script>
+  </body>
+</html>
diff --git a/packages/extension-firefox/src/popup.ts b/packages/extension-firefox/src/popup.ts
new file mode 100644
index 0000000..c3afe82
--- /dev/null
+++ b/packages/extension-firefox/src/popup.ts
@@ -0,0 +1 @@
+import "@gitmarks/extension-shared/popup";
diff --git a/packages/extension-firefox/tsconfig.json b/packages/extension-firefox/tsconfig.json
new file mode 100644
index 0000000..f665ba0
--- /dev/null
+++ b/packages/extension-firefox/tsconfig.json
@@ -0,0 +1,11 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "types": ["chrome", "webextension-polyfill", "node"],
+    "rootDir": "./",
+    "outDir": "./dist-tsc",
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts", "vite.config.ts", "scripts/**/*.mjs"]
+}
diff --git a/packages/extension-firefox/vite.config.ts b/packages/extension-firefox/vite.config.ts
new file mode 100644
index 0000000..e85b581
--- /dev/null
+++ b/packages/extension-firefox/vite.config.ts
@@ -0,0 +1,28 @@
+import { defineConfig } from "vite";
+import { resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = fileURLToPath(new URL(".", import.meta.url));
+
+export default defineConfig({
+  root: "src",
+  build: {
+    outDir: "../dist",
+    emptyOutDir: true,
+    target: "esnext",
+    minify: false,
+    sourcemap: true,
+    rollupOptions: {
+      input: {
+        background: resolve(__dirname, "src/background.ts"),
+        popup: resolve(__dirname, "src/popup.html"),
+        options: resolve(__dirname, "src/options.html"),
+      },
+      output: {
+        entryFileNames: "[name].js",
+        chunkFileNames: "assets/[name]-[hash].js",
+        assetFileNames: "[name].[ext]",
+      },
+    },
+  },
+});
diff --git a/packages/extension-shared/package.json b/packages/extension-shared/package.json
new file mode 100644
index 0000000..bf7eb15
--- /dev/null
+++ b/packages/extension-shared/package.json
@@ -0,0 +1,28 @@
+{
+  "name": "@gitmarks/extension-shared",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts",
+    "./background": "./src/background.ts",
+    "./popup": "./src/popup.ts",
+    "./options": "./src/options.ts"
+  },
+  "scripts": {
+    "typecheck": "tsc -p tsconfig.json --noEmit",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "dependencies": {
+    "@gitmarks/core": "workspace:*",
+    "webextension-polyfill": "^0.12.0",
+    "zod": "^3.23.0"
+  },
+  "devDependencies": {
+    "@types/chrome": "^0.0.268",
+    "@types/webextension-polyfill": "^0.12.0",
+    "jsdom": "^25.0.0",
+    "vitest": "^2.0.0"
+  }
+}
diff --git a/packages/extension-shared/src/background.ts b/packages/extension-shared/src/background.ts
new file mode 100644
index 0000000..d82d434
--- /dev/null
+++ b/packages/extension-shared/src/background.ts
@@ -0,0 +1,121 @@
+import browser from "webextension-polyfill";
+import { GitHubClient } from "@gitmarks/core";
+import { loadSettings, type Settings } from "./lib/settings.js";
+import { getMachineId } from "./lib/machine-id.js";
+import { IdMap } from "./lib/id-mapping.js";
+import { reconcile } from "./lib/reconcile.js";
+import { registerListeners } from "./lib/listeners.js";
+import { applyRemoteChanges } from "./lib/apply-remote.js";
+import { runMaybeReconcile, runPollRemoteOnce, toEtag } from "./lib/background-core.js";
+
+const RECONCILE_INTERVAL_MS = 60 * 60 * 1000;
+const POLL_ALARM_NAME = "gitmarks:poll";
+const RECONCILED_AT_KEY = "gitmarks:lastReconciledAt";
+const LAST_ETAG_KEY = "gitmarks:bookmarksEtag";
+
+let cachedBarId: string | null = null;
+let cachedOtherId: string | null = null;
+
+async function getBarOtherIds(): Promise<{ bar: string; other: string }> {
+  if (cachedBarId != null && cachedOtherId != null) {
+    return { bar: cachedBarId, other: cachedOtherId };
+  }
+  const tree = await browser.bookmarks.getTree();
+  const root = tree[0];
+  if (root?.children == null) {
+    throw new Error("unexpected chrome.bookmarks tree shape");
+  }
+  let bar: string | null = null;
+  let other: string | null = null;
+  for (const child of root.children) {
+    if (child.id === "1") bar = child.id;
+    else if (child.id === "2") other = child.id;
+  }
+  if (bar == null || other == null) {
+    throw new Error("could not find Bookmarks Bar (id=1) or Other Bookmarks (id=2) in tree");
+  }
+  cachedBarId = bar;
+  cachedOtherId = other;
+  return { bar, other };
+}
+
+function buildClient(settings: Settings): GitHubClient {
+  return new GitHubClient({
+    owner: settings.owner,
+    repo: settings.repo,
+    token: settings.token,
+    branch: settings.branch,
+  });
+}
+
+async function maybeReconcile(): Promise<void> {
+  const settings = await loadSettings();
+  if (settings == null) return;
+
+  const stored = await browser.storage.local.get(RECONCILED_AT_KEY);
+  const lastReconciledAt = typeof stored[RECONCILED_AT_KEY] === "number"
+    ? (stored[RECONCILED_AT_KEY] as number)
+    : 0;
+
+  await runMaybeReconcile({
+    now: Date.now(),
+    lastReconciledAt,
+    reconcileIntervalMs: RECONCILE_INTERVAL_MS,
+    runReconcile: async () => {
+      const { bar, other } = await getBarOtherIds();
+      const client = buildClient(settings);
+      const idMap = await IdMap.load();
+      const machineId = await getMachineId();
+      const nowIso = new Date().toISOString();
+      await reconcile(client, idMap, bar, other, machineId, nowIso, settings.stripTrackingParams);
+    },
+    setStorage: (items) => browser.storage.local.set(items),
+    removeStorage: (key) => browser.storage.local.remove(key),
+  });
+}
+
+async function pollRemoteOnce(): Promise<void> {
+  const settings = await loadSettings();
+  if (settings == null) return;
+  const client = buildClient(settings);
+  const stored = await browser.storage.local.get(LAST_ETAG_KEY);
+  const rawEtag = stored[LAST_ETAG_KEY];
+  const etag = typeof rawEtag === "string" ? toEtag(rawEtag) : null;
+
+  await runPollRemoteOnce({
+    etag,
+    now: Date.now(),
+    client,
+    applyRemote: async (data) => {
+      const { bar, other } = await getBarOtherIds();
+      const idMap = await IdMap.load();
+      await applyRemoteChanges(data, idMap, bar, other);
+    },
+    setStorage: (items) => browser.storage.local.set(items),
+    removeStorage: (key) => browser.storage.local.remove(key),
+  });
+}
+
+registerListeners({
+  getClient: async () => {
+    const s = await loadSettings();
+    if (s == null) throw new Error("no settings");
+    return buildClient(s);
+  },
+  getIdMap: async () => IdMap.load(),
+  getBarOtherIds,
+  getMachineId,
+  getStripTrackingParams: async () => {
+    const s = await loadSettings();
+    return s?.stripTrackingParams ?? false;
+  },
+});
+
+browser.alarms.create(POLL_ALARM_NAME, { periodInMinutes: 5 });
+browser.alarms.onAlarm.addListener((alarm) => {
+  if (alarm.name === POLL_ALARM_NAME) {
+    void pollRemoteOnce();
+  }
+});
+
+void maybeReconcile();
diff --git a/packages/extension-shared/src/index.ts b/packages/extension-shared/src/index.ts
new file mode 100644
index 0000000..5939263
--- /dev/null
+++ b/packages/extension-shared/src/index.ts
@@ -0,0 +1,55 @@
+// Re-exports the public surface that browser-specific shells consume.
+// Kept alphabetized; add new lib/ surfaces here as they appear.
+
+export { applyRemoteChanges } from "./lib/apply-remote.js";
+export * from "./lib/background-core.js";
+export { buildBookmark, type BuildBookmarkInput } from "./lib/bookmark-factory.js";
+export {
+  BOOKMARKS_PATH,
+  emptyBookmarksFile,
+  updateBookmarksOrBootstrap,
+} from "./lib/bookmarks-file.js";
+export {
+  BOOKMARKS_BAR_FOLDER,
+  OTHER_BOOKMARKS_FOLDER,
+  folderPathFromNode,
+  splitFolderPath,
+  type SplitPath,
+  type TreeNode,
+} from "./lib/folder-path.js";
+export {
+  IdMap,
+  asNodeId,
+  asUlid,
+  type NodeId,
+  type Ulid,
+} from "./lib/id-mapping.js";
+export {
+  flushPending,
+  registerListeners,
+  __resetForTest,
+  type ListenerDeps,
+} from "./lib/listeners.js";
+export { getMachineId } from "./lib/machine-id.js";
+export { reconcile } from "./lib/reconcile.js";
+export {
+  saveBookmark,
+  type PageInfo,
+  type SaveOptions,
+  type SaveResult,
+} from "./lib/save-flow.js";
+export {
+  SettingsCorruptError,
+  clearSettings,
+  loadSettings,
+  saveSettings,
+  settingsSchema,
+  type Settings,
+} from "./lib/settings.js";
+export {
+  clearSuppression,
+  isNodeSuppressed,
+  isSuppressed,
+  suppress,
+  suppressNode,
+} from "./lib/suppression.js";
diff --git a/packages/extension-chrome/src/lib/apply-remote.ts b/packages/extension-shared/src/lib/apply-remote.ts
similarity index 91%
rename from packages/extension-chrome/src/lib/apply-remote.ts
rename to packages/extension-shared/src/lib/apply-remote.ts
index 2583ae5..05d1a47 100644
--- a/packages/extension-chrome/src/lib/apply-remote.ts
+++ b/packages/extension-shared/src/lib/apply-remote.ts
@@ -1,3 +1,4 @@
+import browser from "webextension-polyfill";
 import type { BookmarksFile } from "@gitmarks/core";
 import { type IdMap, asUlid, asNodeId } from "./id-mapping.js";
 import { splitFolderPath } from "./folder-path.js";
@@ -17,7 +18,7 @@ export async function applyRemoteChanges(
         if (existingNode != null) {
           suppress(bm.url);
           try {
-            await chrome.bookmarks.remove(existingNode);
+            await browser.bookmarks.remove(existingNode);
             idMap.removeByUlid(asUlid(bm.id));
           } catch (err) {
             const msg = err instanceof Error ? err.message : String(err);
@@ -51,7 +52,7 @@ export async function applyRemoteChanges(
         otherBookmarksId,
       );
       suppress(bm.url);
-      const created = await chrome.bookmarks.create({
+      const created = await browser.bookmarks.create({
         parentId,
         title: bm.title,
         url: bm.url,
@@ -73,7 +74,7 @@ async function applyRemoteEdit(
 ): Promise<void> {
   let current: chrome.bookmarks.BookmarkTreeNode | undefined;
   try {
-    const found = await chrome.bookmarks.get(nodeId);
+    const found = await browser.bookmarks.get(nodeId);
     current = found[0];
   } catch (err) {
     const msg = err instanceof Error ? err.message : String(err);
@@ -105,7 +106,7 @@ async function applyRemoteEdit(
   suppress(remoteUrl);
   suppressNode(nodeId);
 
-  await chrome.bookmarks.update(nodeId, changes);
+  await browser.bookmarks.update(nodeId, changes);
 }
 
 async function ensureFolderPath(
@@ -122,13 +123,13 @@ async function ensureFolderPath(
 }
 
 async function ensureSubfolder(parentId: string, title: string): Promise<string> {
-  const children = await chrome.bookmarks.getSubTree(parentId);
+  const children = await browser.bookmarks.getSubTree(parentId);
   const parent = children[0];
   if (parent?.children != null) {
     for (const child of parent.children) {
       if (child.url == null && child.title === title) return child.id;
     }
   }
-  const folder = await chrome.bookmarks.create({ parentId, title });
+  const folder = await browser.bookmarks.create({ parentId, title });
   return folder.id;
 }
diff --git a/packages/extension-chrome/src/lib/background-core.ts b/packages/extension-shared/src/lib/background-core.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/background-core.ts
rename to packages/extension-shared/src/lib/background-core.ts
diff --git a/packages/extension-chrome/src/lib/bookmark-factory.ts b/packages/extension-shared/src/lib/bookmark-factory.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/bookmark-factory.ts
rename to packages/extension-shared/src/lib/bookmark-factory.ts
diff --git a/packages/extension-chrome/src/lib/bookmarks-file.ts b/packages/extension-shared/src/lib/bookmarks-file.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/bookmarks-file.ts
rename to packages/extension-shared/src/lib/bookmarks-file.ts
diff --git a/packages/extension-chrome/src/lib/folder-path.ts b/packages/extension-shared/src/lib/folder-path.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/folder-path.ts
rename to packages/extension-shared/src/lib/folder-path.ts
diff --git a/packages/extension-chrome/src/lib/id-mapping.ts b/packages/extension-shared/src/lib/id-mapping.ts
similarity index 94%
rename from packages/extension-chrome/src/lib/id-mapping.ts
rename to packages/extension-shared/src/lib/id-mapping.ts
index f207865..35e1b9c 100644
--- a/packages/extension-chrome/src/lib/id-mapping.ts
+++ b/packages/extension-shared/src/lib/id-mapping.ts
@@ -1,3 +1,5 @@
+import browser from "webextension-polyfill";
+
 // Branded string types — nominal phantom types that prevent argument-swap bugs.
 declare const ulidBrand: unique symbol;
 declare const nodeIdBrand: unique symbol;
@@ -35,7 +37,7 @@ export class IdMap {
   }
 
   static async load(): Promise<IdMap> {
-    const stored = await chrome.storage.local.get(KEY);
+    const stored = await browser.storage.local.get(KEY);
     const raw = stored[KEY];
     const map = new IdMap();
     if (raw == null || typeof raw !== "object") return map;
@@ -50,7 +52,7 @@ export class IdMap {
 
   async save(): Promise<void> {
     const entries = Array.from(this.#ulidToNode.entries());
-    await chrome.storage.local.set({ [KEY]: { entries } });
+    await browser.storage.local.set({ [KEY]: { entries } });
   }
 
   /**
diff --git a/packages/extension-chrome/src/lib/listeners.ts b/packages/extension-shared/src/lib/listeners.ts
similarity index 96%
rename from packages/extension-chrome/src/lib/listeners.ts
rename to packages/extension-shared/src/lib/listeners.ts
index 7ef2992..44c9d1a 100644
--- a/packages/extension-chrome/src/lib/listeners.ts
+++ b/packages/extension-shared/src/lib/listeners.ts
@@ -1,3 +1,4 @@
+import browser from "webextension-polyfill";
 import type {
   Bookmark,
   BookmarksFile,
@@ -54,10 +55,10 @@ export function __resetForTest(): void {
 
 export function registerListeners(d: ListenerDeps): void {
   deps = d;
-  chrome.bookmarks.onCreated.addListener(onCreated);
-  chrome.bookmarks.onChanged.addListener(onChanged);
-  chrome.bookmarks.onMoved.addListener(onMoved);
-  chrome.bookmarks.onRemoved.addListener(onRemoved);
+  browser.bookmarks.onCreated.addListener(onCreated);
+  browser.bookmarks.onChanged.addListener(onChanged);
+  browser.bookmarks.onMoved.addListener(onMoved);
+  browser.bookmarks.onRemoved.addListener(onRemoved);
 }
 
 function schedule(): void {
@@ -80,7 +81,7 @@ async function runFlush(): Promise<void> {
   try {
     await flushPending();
     consecutiveFailures = 0;
-    await chrome.storage.local.remove(LAST_ERROR_KEY);
+    await browser.storage.local.remove(LAST_ERROR_KEY);
   } catch (err) {
     consecutiveFailures += 1;
     console.error(
@@ -93,7 +94,7 @@ async function runFlush(): Promise<void> {
       source: "flush",
       kind: "unknown",
     };
-    await chrome.storage.local.set({ [LAST_ERROR_KEY]: record });
+    await browser.storage.local.set({ [LAST_ERROR_KEY]: record });
   } finally {
     flushing = false;
     if (pendingReschedule || pending.length > 0) {
diff --git a/packages/extension-chrome/src/lib/machine-id.ts b/packages/extension-shared/src/lib/machine-id.ts
similarity index 77%
rename from packages/extension-chrome/src/lib/machine-id.ts
rename to packages/extension-shared/src/lib/machine-id.ts
index 8d0369c..d363160 100644
--- a/packages/extension-chrome/src/lib/machine-id.ts
+++ b/packages/extension-shared/src/lib/machine-id.ts
@@ -1,3 +1,5 @@
+import browser from "webextension-polyfill";
+
 const KEY = "gitmarks:machineId";
 const ALPHABET = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"; // Crockford base32
 
@@ -10,12 +12,12 @@ function newId(): string {
 }
 
 export async function getMachineId(): Promise<string> {
-  const stored = await chrome.storage.local.get(KEY);
+  const stored = await browser.storage.local.get(KEY);
   const existing = stored[KEY];
   if (typeof existing === "string" && /^[0-9A-HJKMNP-TV-Z]{8}$/.test(existing)) {
     return existing;
   }
   const fresh = newId();
-  await chrome.storage.local.set({ [KEY]: fresh });
+  await browser.storage.local.set({ [KEY]: fresh });
   return fresh;
 }
diff --git a/packages/extension-chrome/src/lib/reconcile.ts b/packages/extension-shared/src/lib/reconcile.ts
similarity index 97%
rename from packages/extension-chrome/src/lib/reconcile.ts
rename to packages/extension-shared/src/lib/reconcile.ts
index e1d4385..34f8185 100644
--- a/packages/extension-chrome/src/lib/reconcile.ts
+++ b/packages/extension-shared/src/lib/reconcile.ts
@@ -1,3 +1,4 @@
+import browser from "webextension-polyfill";
 import type {
   BookmarksFile,
   Bookmark,
@@ -109,7 +110,7 @@ async function collectLocalBookmarks(
   otherBookmarksId: string,
 ): Promise<Map<string, LocalEntry>> {
   const out = new Map<string, LocalEntry>();
-  const tree = await chrome.bookmarks.getTree();
+  const tree = await browser.bookmarks.getTree();
   if (tree[0]?.children == null) return out;
 
   for (const top of tree[0].children) {
diff --git a/packages/extension-chrome/src/lib/save-flow.ts b/packages/extension-shared/src/lib/save-flow.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/save-flow.ts
rename to packages/extension-shared/src/lib/save-flow.ts
diff --git a/packages/extension-chrome/src/lib/settings.ts b/packages/extension-shared/src/lib/settings.ts
similarity index 84%
rename from packages/extension-chrome/src/lib/settings.ts
rename to packages/extension-shared/src/lib/settings.ts
index 7dd7854..b9d9888 100644
--- a/packages/extension-chrome/src/lib/settings.ts
+++ b/packages/extension-shared/src/lib/settings.ts
@@ -1,3 +1,4 @@
+import browser from "webextension-polyfill";
 import { z } from "zod";
 
 const SETTINGS_KEY = "gitmarks:settings";
@@ -20,7 +21,7 @@ export class SettingsCorruptError extends Error {
 }
 
 export async function loadSettings(): Promise<Settings | null> {
-  const result = await chrome.storage.local.get(SETTINGS_KEY);
+  const result = await browser.storage.local.get(SETTINGS_KEY);
   const raw = result[SETTINGS_KEY];
   if (raw == null) return null;
   const parsed = settingsSchema.safeParse(raw);
@@ -37,9 +38,9 @@ export async function loadSettings(): Promise<Settings | null> {
 
 export async function saveSettings(value: Settings): Promise<void> {
   const validated = settingsSchema.parse(value);
-  await chrome.storage.local.set({ [SETTINGS_KEY]: validated });
+  await browser.storage.local.set({ [SETTINGS_KEY]: validated });
 }
 
 export async function clearSettings(): Promise<void> {
-  await chrome.storage.local.remove(SETTINGS_KEY);
+  await browser.storage.local.remove(SETTINGS_KEY);
 }
diff --git a/packages/extension-chrome/src/lib/suppression.ts b/packages/extension-shared/src/lib/suppression.ts
similarity index 100%
rename from packages/extension-chrome/src/lib/suppression.ts
rename to packages/extension-shared/src/lib/suppression.ts
diff --git a/packages/extension-shared/src/options.html b/packages/extension-shared/src/options.html
new file mode 100644
index 0000000..16309e7
--- /dev/null
+++ b/packages/extension-shared/src/options.html
@@ -0,0 +1,64 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks — settings</title>
+    <style>
+      body {
+        font-family: system-ui, sans-serif;
+        max-width: 480px;
+        margin: 2rem auto;
+        padding: 0 1rem;
+        color: #1a1a1a;
+        background: #fafafa;
+      }
+      h1 { font-size: 1.5rem; margin-bottom: 1.5rem; }
+      label { display: block; margin-bottom: 1rem; }
+      label > span { display: block; font-size: 0.85rem; margin-bottom: 0.25rem; }
+      input { width: 100%; padding: 0.5rem; font: inherit; box-sizing: border-box; border: 1px solid #ccc; border-radius: 4px; }
+      .row { display: flex; gap: 0.5rem; margin-top: 1.5rem; }
+      button { padding: 0.5rem 1rem; font: inherit; border: 1px solid #444; background: #222; color: white; border-radius: 4px; cursor: pointer; }
+      button.secondary { background: white; color: #222; }
+      #status { margin-top: 1rem; font-size: 0.9rem; min-height: 1.2em; }
+      #status.ok { color: #096; }
+      #status.err { color: #c00; }
+    </style>
+  </head>
+  <body>
+    <h1>gitmarks settings</h1>
+
+    <label>
+      <span>GitHub fine-grained personal access token</span>
+      <input id="token" type="password" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Repo owner (your GitHub login)</span>
+      <input id="owner" type="text" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Repo name</span>
+      <input id="repo" type="text" autocomplete="off" spellcheck="false" />
+    </label>
+
+    <label>
+      <span>Branch</span>
+      <input id="branch" type="text" autocomplete="off" spellcheck="false" value="main" />
+    </label>
+
+    <label class="checkbox">
+      <input id="stripTrackingParams" type="checkbox" />
+      Strip tracking parameters (utm_*, fbclid, gclid, msclkid, mc_*) at save time
+    </label>
+
+    <div class="row">
+      <button id="validate" class="secondary">Validate</button>
+      <button id="save">Save</button>
+    </div>
+
+    <p id="status"></p>
+
+    <script type="module" src="./options.ts"></script>
+  </body>
+</html>
diff --git a/packages/extension-shared/src/options.ts b/packages/extension-shared/src/options.ts
new file mode 100644
index 0000000..595dbd4
--- /dev/null
+++ b/packages/extension-shared/src/options.ts
@@ -0,0 +1,118 @@
+import browser from "webextension-polyfill";
+import {
+  GitHubClient,
+  GitHubAuthError,
+  GitHubError,
+  GitHubNotFoundError,
+} from "@gitmarks/core";
+import { loadSettings, saveSettings, SettingsCorruptError, type Settings } from "./lib/settings.js";
+
+const $ = <T extends HTMLElement>(id: string): T => {
+  const el = document.getElementById(id);
+  if (el == null) throw new Error(`#${id} not found`);
+  return el as T;
+};
+
+const tokenInput = $<HTMLInputElement>("token");
+const ownerInput = $<HTMLInputElement>("owner");
+const repoInput = $<HTMLInputElement>("repo");
+const branchInput = $<HTMLInputElement>("branch");
+const stripTrackingParamsInput = $<HTMLInputElement>("stripTrackingParams");
+const validateBtn = $<HTMLButtonElement>("validate");
+const saveBtn = $<HTMLButtonElement>("save");
+const status = $<HTMLParagraphElement>("status");
+
+function readForm(): Settings {
+  return {
+    token: tokenInput.value.trim(),
+    owner: ownerInput.value.trim(),
+    repo: repoInput.value.trim(),
+    branch: branchInput.value.trim() || "main",
+    stripTrackingParams: stripTrackingParamsInput.checked,
+  };
+}
+
+function setStatus(msg: string, kind: "ok" | "err" | "neutral"): void {
+  status.textContent = msg;
+  status.className = kind === "neutral" ? "" : kind;
+}
+
+async function loadIntoForm(): Promise<void> {
+  let s;
+  try {
+    s = await loadSettings();
+  } catch (err) {
+    if (err instanceof SettingsCorruptError) {
+      // Clear all form fields so the user can re-enter valid settings.
+      tokenInput.value = "";
+      ownerInput.value = "";
+      repoInput.value = "";
+      branchInput.value = "";
+      setStatus("Stored settings are corrupted — please reconfigure.", "err");
+      return;
+    }
+    throw err;
+  }
+  if (s == null) return;
+  tokenInput.value = s.token;
+  ownerInput.value = s.owner;
+  repoInput.value = s.repo;
+  branchInput.value = s.branch;
+  stripTrackingParamsInput.checked = s.stripTrackingParams;
+}
+
+validateBtn.addEventListener("click", async () => {
+  setStatus("validating…", "neutral");
+  let s: Settings;
+  try {
+    s = readForm();
+  } catch (err) {
+    setStatus(err instanceof Error ? err.message : String(err), "err");
+    return;
+  }
+  const client = new GitHubClient(s);
+  try {
+    await client.read("bookmarks.json");
+    setStatus("✓ valid PAT, repo exists, bookmarks.json found", "ok");
+  } catch (err) {
+    if (err instanceof GitHubNotFoundError) {
+      setStatus(
+        "✓ valid PAT, repo exists (bookmarks.json not yet created — will be on first save)",
+        "ok",
+      );
+      return;
+    }
+    console.error("[gitmarks] validate failed", err);
+    if (err instanceof GitHubAuthError) {
+      setStatus(
+        "PAT rejected — check the token is valid and has 'Contents: Read and write' scope on this repo.",
+        "err",
+      );
+      return;
+    }
+    if (err instanceof GitHubError && err.status >= 500) {
+      setStatus(
+        `GitHub is having issues (${err.status}). Try again in a minute.`,
+        "err",
+      );
+      return;
+    }
+    if (err instanceof Error && (err.message.includes("Failed to fetch") || err.message.includes("NetworkError"))) {
+      setStatus("Network error — check your connection and try again.", "err");
+      return;
+    }
+    setStatus(err instanceof Error ? err.message : String(err), "err");
+  }
+});
+
+saveBtn.addEventListener("click", async () => {
+  try {
+    await saveSettings(readForm());
+    setStatus("✓ saved", "ok");
+  } catch (err) {
+    console.error("[gitmarks] save settings failed", err);
+    setStatus(err instanceof Error ? err.message : String(err), "err");
+  }
+});
+
+void loadIntoForm();
diff --git a/packages/extension-shared/src/popup.html b/packages/extension-shared/src/popup.html
new file mode 100644
index 0000000..69a0b72
--- /dev/null
+++ b/packages/extension-shared/src/popup.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <title>gitmarks</title>
+    <style>
+      body {
+        font-family: system-ui, sans-serif;
+        margin: 0;
+        padding: 0;
+        background: #fafafa;
+        color: #1a1a1a;
+        min-width: 240px;
+      }
+      main { padding: 1rem; }
+      button {
+        width: 100%;
+        padding: 0.6rem;
+        font: inherit;
+        border: 1px solid #444;
+        background: #222;
+        color: white;
+        border-radius: 4px;
+        cursor: pointer;
+      }
+      button:disabled { opacity: 0.6; cursor: progress; }
+      #status { margin-top: 0.75rem; font-size: 0.85rem; min-height: 1.2em; }
+      #status.ok { color: #096; }
+      #status.err { color: #c00; }
+      .title { font-size: 0.85rem; color: #555; margin: 0 0 0.5rem 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+    </style>
+  </head>
+  <body>
+    <main id="root">loading…</main>
+    <script type="module" src="./popup.ts"></script>
+  </body>
+</html>
diff --git a/packages/extension-shared/src/popup.ts b/packages/extension-shared/src/popup.ts
new file mode 100644
index 0000000..9e84815
--- /dev/null
+++ b/packages/extension-shared/src/popup.ts
@@ -0,0 +1,135 @@
+import browser from "webextension-polyfill";
+import { GitHubClient } from "@gitmarks/core";
+import { loadSettings, SettingsCorruptError } from "./lib/settings.js";
+import { getMachineId } from "./lib/machine-id.js";
+import { saveBookmark, type SaveResult } from "./lib/save-flow.js";
+import type { LastErrorRecord } from "./lib/background-core.js";
+
+const root = document.getElementById("root");
+if (root == null) throw new Error("#root not found");
+
+async function getActiveTab(): Promise<browser.Tabs.Tab | null> {
+  // When opened as a real extension popup, currentWindow refers to the
+  // browser window the user was in (not the popup's own floating window),
+  // so this returns the tab they were viewing. activeTab grants access to
+  // title + url for that one tab on user-gesture popup open; no broader
+  // tabs permission is required.
+  const [tab] = await browser.tabs.query({ active: true, currentWindow: true });
+  if (tab != null && tab.url != null && !tab.url.startsWith("chrome-extension://")) {
+    return tab;
+  }
+  return null;
+}
+
+async function render(): Promise<void> {
+  let settings;
+  try {
+    settings = await loadSettings();
+  } catch (err) {
+    if (err instanceof SettingsCorruptError) {
+      root!.innerHTML = `<p class="err">Settings are corrupted — please reconfigure gitmarks.</p>
+        <button id="setup">Open settings</button>`;
+      document.getElementById("setup")?.addEventListener("click", () => {
+        browser.runtime.openOptionsPage();
+        window.close();
+      });
+      return;
+    }
+    throw err;
+  }
+  if (settings == null) {
+    root!.innerHTML = `
+      <p class="title">Welcome to gitmarks.</p>
+      <button id="setup">Set up gitmarks</button>
+    `;
+    document.getElementById("setup")!.addEventListener("click", () => {
+      browser.runtime.openOptionsPage();
+      window.close();
+    });
+    return;
+  }
+
+  const tab = await getActiveTab();
+  if (tab == null || tab.url == null) {
+    root!.innerHTML = `<p id="status" class="err">No active tab.</p>`;
+    return;
+  }
+
+  root!.innerHTML = `
+    <p class="title" title="${escapeAttr(tab.title ?? tab.url)}">${escapeText(tab.title ?? tab.url)}</p>
+    <button id="save">Save this page</button>
+    <p id="status"></p>
+  `;
+
+  const errStored = await browser.storage.local.get("gitmarks:lastError");
+  const lastErr = errStored["gitmarks:lastError"] as LastErrorRecord | undefined;
+  if (lastErr != null) {
+    const banner = document.createElement("p");
+    banner.className = "err";
+    banner.style.fontSize = "0.8rem";
+    banner.style.marginTop = "0.5rem";
+    const label = lastErr.kind === "auth" ? "Background sync auth failed" : `Background ${lastErr.source} failed`;
+    banner.textContent = `${label}: ${lastErr.message}`;
+    root!.appendChild(banner);
+  }
+
+  const saveBtn = document.getElementById("save") as HTMLButtonElement;
+  const status = document.getElementById("status")!;
+
+  saveBtn.addEventListener("click", async () => {
+    saveBtn.disabled = true;
+    saveBtn.textContent = "saving…";
+    status.className = "";
+    status.textContent = "";
+    let result: SaveResult;
+    try {
+      const machineId = await getMachineId();
+      const client = new GitHubClient({
+        owner: settings.owner,
+        repo: settings.repo,
+        token: settings.token,
+        branch: settings.branch,
+      });
+      result = await saveBookmark(
+        client,
+        { url: tab.url!, title: tab.title ?? tab.url! },
+        machineId,
+        new Date().toISOString(),
+        { stripTrackingParams: settings.stripTrackingParams },
+      );
+    } catch (err) {
+      result = {
+        ok: false,
+        kind: "unknown",
+        message: err instanceof Error ? err.message : String(err),
+      };
+    }
+    if (result.ok) {
+      status.className = "ok";
+      status.textContent = "✓ saved";
+    } else {
+      status.className = "err";
+      status.textContent = result.message;
+      saveBtn.disabled = false;
+      saveBtn.textContent = "Try again";
+    }
+  });
+}
+
+function escapeText(s: string): string {
+  return s
+    .replace(/&/g, "&amp;")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;");
+}
+
+function escapeAttr(s: string): string {
+  return escapeText(s).replace(/"/g, "&quot;");
+}
+
+render().catch((err) => {
+  console.error("[gitmarks] popup render failed", err);
+  if (root != null) {
+    root.innerHTML = `<p class="err">Something went wrong opening gitmarks. Please reload the extension.</p>`;
+  }
+});
diff --git a/packages/extension-chrome/test/apply-remote.test.ts b/packages/extension-shared/test/apply-remote.test.ts
similarity index 100%
rename from packages/extension-chrome/test/apply-remote.test.ts
rename to packages/extension-shared/test/apply-remote.test.ts
diff --git a/packages/extension-chrome/test/background-core.test.ts b/packages/extension-shared/test/background-core.test.ts
similarity index 100%
rename from packages/extension-chrome/test/background-core.test.ts
rename to packages/extension-shared/test/background-core.test.ts
diff --git a/packages/extension-chrome/test/bookmark-factory.test.ts b/packages/extension-shared/test/bookmark-factory.test.ts
similarity index 100%
rename from packages/extension-chrome/test/bookmark-factory.test.ts
rename to packages/extension-shared/test/bookmark-factory.test.ts
diff --git a/packages/extension-chrome/test/bookmarks-file.test.ts b/packages/extension-shared/test/bookmarks-file.test.ts
similarity index 100%
rename from packages/extension-chrome/test/bookmarks-file.test.ts
rename to packages/extension-shared/test/bookmarks-file.test.ts
diff --git a/packages/extension-chrome/test/folder-path.test.ts b/packages/extension-shared/test/folder-path.test.ts
similarity index 100%
rename from packages/extension-chrome/test/folder-path.test.ts
rename to packages/extension-shared/test/folder-path.test.ts
diff --git a/packages/extension-chrome/test/id-mapping.test.ts b/packages/extension-shared/test/id-mapping.test.ts
similarity index 100%
rename from packages/extension-chrome/test/id-mapping.test.ts
rename to packages/extension-shared/test/id-mapping.test.ts
diff --git a/packages/extension-chrome/test/listeners.test.ts b/packages/extension-shared/test/listeners.test.ts
similarity index 100%
rename from packages/extension-chrome/test/listeners.test.ts
rename to packages/extension-shared/test/listeners.test.ts
diff --git a/packages/extension-chrome/test/machine-id.test.ts b/packages/extension-shared/test/machine-id.test.ts
similarity index 100%
rename from packages/extension-chrome/test/machine-id.test.ts
rename to packages/extension-shared/test/machine-id.test.ts
diff --git a/packages/extension-chrome/test/reconcile.test.ts b/packages/extension-shared/test/reconcile.test.ts
similarity index 100%
rename from packages/extension-chrome/test/reconcile.test.ts
rename to packages/extension-shared/test/reconcile.test.ts
diff --git a/packages/extension-chrome/test/save-flow.test.ts b/packages/extension-shared/test/save-flow.test.ts
similarity index 100%
rename from packages/extension-chrome/test/save-flow.test.ts
rename to packages/extension-shared/test/save-flow.test.ts
diff --git a/packages/extension-chrome/test/settings.test.ts b/packages/extension-shared/test/settings.test.ts
similarity index 100%
rename from packages/extension-chrome/test/settings.test.ts
rename to packages/extension-shared/test/settings.test.ts
diff --git a/packages/extension-chrome/test/setup.ts b/packages/extension-shared/test/setup.ts
similarity index 98%
rename from packages/extension-chrome/test/setup.ts
rename to packages/extension-shared/test/setup.ts
index 244362e..481e5e5 100644
--- a/packages/extension-chrome/test/setup.ts
+++ b/packages/extension-shared/test/setup.ts
@@ -62,6 +62,7 @@ const chromeStub = {
 };
 
 vi.stubGlobal("chrome", chromeStub);
+vi.stubGlobal("browser", chromeStub);
 
 beforeEach(async () => {
   await chromeStub.storage.local.clear();
diff --git a/packages/extension-chrome/test/suppression.test.ts b/packages/extension-shared/test/suppression.test.ts
similarity index 100%
rename from packages/extension-chrome/test/suppression.test.ts
rename to packages/extension-shared/test/suppression.test.ts
diff --git a/packages/extension-shared/test/webextension-polyfill-stub.ts b/packages/extension-shared/test/webextension-polyfill-stub.ts
new file mode 100644
index 0000000..a775d54
--- /dev/null
+++ b/packages/extension-shared/test/webextension-polyfill-stub.ts
@@ -0,0 +1,25 @@
+/**
+ * Test-environment stub for webextension-polyfill.
+ *
+ * The real polyfill throws "This script should only be loaded in a browser
+ * extension" when imported in Node/jsdom. We redirect the import here via
+ * the vitest alias so source files that do
+ *
+ *   import browser from "webextension-polyfill";
+ *
+ * receive the same chromeStub object that test/setup.ts installs as
+ * globalThis.browser (and globalThis.chrome).  The Proxy defers the lookup
+ * until actual property access, so it works even though setup.ts runs its
+ * vi.stubGlobal calls slightly after module evaluation order.
+ */
+const browserProxy = new Proxy({} as Record<string | symbol, unknown>, {
+  get(_target, prop) {
+    const g = (globalThis as Record<string, unknown>)["browser"];
+    if (g != null) {
+      return (g as Record<string | symbol, unknown>)[prop];
+    }
+    return undefined;
+  },
+});
+
+export default browserProxy;
diff --git a/packages/extension-shared/tsconfig.json b/packages/extension-shared/tsconfig.json
new file mode 100644
index 0000000..7a3e845
--- /dev/null
+++ b/packages/extension-shared/tsconfig.json
@@ -0,0 +1,11 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "types": ["chrome"],
+    "rootDir": "./",
+    "outDir": "./dist-tsc",
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts", "test/**/*.ts", "vitest.config.ts"]
+}
diff --git a/packages/extension-shared/vitest.config.ts b/packages/extension-shared/vitest.config.ts
new file mode 100644
index 0000000..c8e7fbf
--- /dev/null
+++ b/packages/extension-shared/vitest.config.ts
@@ -0,0 +1,21 @@
+import { defineConfig } from "vitest/config";
+import path from "node:path";
+
+export default defineConfig({
+  resolve: {
+    alias: {
+      // Redirect "webextension-polyfill" to a test stub so the real polyfill
+      // (which throws in Node/jsdom) is never loaded during unit tests.
+      "webextension-polyfill": path.resolve(
+        __dirname,
+        "test/webextension-polyfill-stub.ts",
+      ),
+    },
+  },
+  test: {
+    environment: "jsdom",
+    include: ["test/**/*.test.ts"],
+    setupFiles: ["./test/setup.ts"],
+    globals: false,
+  },
+});
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 139ba10..2f12137 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -23,35 +23,79 @@ importers:
     devDependencies:
       vitest:
         specifier: ^2.0.0
-        version: 2.1.9(jsdom@25.0.1)
+        version: 2.1.9(@types/node@22.19.19)(jsdom@25.0.1)
 
   packages/extension-chrome:
     dependencies:
       '@gitmarks/core':
         specifier: workspace:*
         version: link:../core
+      '@gitmarks/extension-shared':
+        specifier: workspace:*
+        version: link:../extension-shared
       zod:
         specifier: ^3.23.0
         version: 3.25.76
     devDependencies:
       '@crxjs/vite-plugin':
         specifier: ^2.4.0
-        version: 2.4.0(vite@5.4.21)
+        version: 2.4.0(vite@5.4.21(@types/node@22.19.19))
       '@playwright/test':
         specifier: ^1.48.0
         version: 1.60.0
       '@types/chrome':
         specifier: ^0.0.268
         version: 0.0.268
+      vite:
+        specifier: ^5.4.0
+        version: 5.4.21(@types/node@22.19.19)
+
+  packages/extension-firefox:
+    dependencies:
+      '@gitmarks/core':
+        specifier: workspace:*
+        version: link:../core
+      '@gitmarks/extension-shared':
+        specifier: workspace:*
+        version: link:../extension-shared
+    devDependencies:
+      '@types/chrome':
+        specifier: ^0.0.268
+        version: 0.0.268
+      '@types/node':
+        specifier: ^22.0.0
+        version: 22.19.19
+      '@types/webextension-polyfill':
+        specifier: ^0.12.0
+        version: 0.12.5
+      vite:
+        specifier: ^5.4.0
+        version: 5.4.21(@types/node@22.19.19)
+
+  packages/extension-shared:
+    dependencies:
+      '@gitmarks/core':
+        specifier: workspace:*
+        version: link:../core
+      webextension-polyfill:
+        specifier: ^0.12.0
+        version: 0.12.0
+      zod:
+        specifier: ^3.23.0
+        version: 3.25.76
+    devDependencies:
+      '@types/chrome':
+        specifier: ^0.0.268
+        version: 0.0.268
+      '@types/webextension-polyfill':
+        specifier: ^0.12.0
+        version: 0.12.5
       jsdom:
         specifier: ^25.0.0
         version: 25.0.1
-      vite:
-        specifier: ^5.4.0
-        version: 5.4.21
       vitest:
         specifier: ^2.0.0
-        version: 2.1.9(jsdom@25.0.1)
+        version: 2.1.9(@types/node@22.19.19)(jsdom@25.0.1)
 
 packages:
 
@@ -396,6 +440,12 @@ packages:
   '@types/har-format@1.2.16':
     resolution: {integrity: sha512-fluxdy7ryD3MV6h8pTfTYpy/xQzCFC7m89nOH9y94cNqJ1mDIDPut7MnRHI3F6qRmh/cT2fUjG1MLdCNb4hE9A==}
 
+  '@types/node@22.19.19':
+    resolution: {integrity: sha512-dyh/xO2Fh5bYrfWaaqGrRQQGkNdmYw6AmaAUvYeUMNTWQtvb796ikLdmTchRmOlOiIJ1TDXfWgVx1QkUlQ6Hew==}
+
+  '@types/webextension-polyfill@0.12.5':
+    resolution: {integrity: sha512-uKSAv6LgcVdINmxXMKBuVIcg/2m5JZugoZO8x20g7j2bXJkPIl/lVGQcDlbV+aXAiTyXT2RA5U5mI4IGCDMQeg==}
+
   '@vitest/expect@2.1.9':
     resolution: {integrity: sha512-UJCIkTBenHeKT1TTlKMJWy1laZewsRIzYighyYiJKZreqtdxSos/S1t+ktRMQWu2CKqaarrkeszJx1cgC5tGZw==}
 
@@ -879,6 +929,9 @@ packages:
     resolution: {integrity: sha512-fIRiVTJNcSRmXKPZtGzFQv9WRrZ3M9eoptl/teFJvjOzmpU+/K/JH6HZ8deBfb5vMEpicJcLn7JmvdknlMq7Zg==}
     hasBin: true
 
+  undici-types@6.21.0:
+    resolution: {integrity: sha512-iwDZqg0QAGrg9Rav5H4n0M64c3mkR59cJ6wQp+7C4nI0gsmExaedaYLNO44eT4AtBBwjbTiGPMlt2Md0T9H9JQ==}
+
   universalify@2.0.1:
     resolution: {integrity: sha512-gptHNQghINnc/vTGIk0SOFGFNXw7JVrlRUtConJRlvaw6DuX0wO5Jeko9sWrMBhh+PsYAZ7oXAiOnf/UKogyiw==}
     engines: {node: '>= 10.0.0'}
@@ -948,6 +1001,9 @@ packages:
     resolution: {integrity: sha512-o8qghlI8NZHU1lLPrpi2+Uq7abh4GGPpYANlalzWxyWteJOCsr/P+oPBA49TOLu5FTZO4d3F9MnWJfiMo4BkmA==}
     engines: {node: '>=18'}
 
+  webextension-polyfill@0.12.0:
+    resolution: {integrity: sha512-97TBmpoWJEE+3nFBQ4VocyCdLKfw54rFaJ6EVQYLBCXqCIpLSZkwGgASpv4oPt9gdKCJ80RJlcmNzNn008Ag6Q==}
+
   webidl-conversions@7.0.0:
     resolution: {integrity: sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==}
     engines: {node: '>=12'}
@@ -1002,7 +1058,7 @@ snapshots:
       '@csstools/css-tokenizer': 3.0.4
       lru-cache: 10.4.3
 
-  '@crxjs/vite-plugin@2.4.0(vite@5.4.21)':
+  '@crxjs/vite-plugin@2.4.0(vite@5.4.21(@types/node@22.19.19))':
     dependencies:
       '@rollup/pluginutils': 4.2.1
       '@webcomponents/custom-elements': 1.6.0
@@ -1020,7 +1076,7 @@ snapshots:
       react-refresh: 0.13.0
       rollup: 2.79.2
       rxjs: 7.5.7
-      vite: 5.4.21
+      vite: 5.4.21(@types/node@22.19.19)
     transitivePeerDependencies:
       - supports-color
 
@@ -1228,6 +1284,12 @@ snapshots:
 
   '@types/har-format@1.2.16': {}
 
+  '@types/node@22.19.19':
+    dependencies:
+      undici-types: 6.21.0
+
+  '@types/webextension-polyfill@0.12.5': {}
+
   '@vitest/expect@2.1.9':
     dependencies:
       '@vitest/spy': 2.1.9
@@ -1235,13 +1297,13 @@ snapshots:
       chai: 5.3.3
       tinyrainbow: 1.2.0
 
-  '@vitest/mocker@2.1.9(vite@5.4.21)':
+  '@vitest/mocker@2.1.9(vite@5.4.21(@types/node@22.19.19))':
     dependencies:
       '@vitest/spy': 2.1.9
       estree-walker: 3.0.3
       magic-string: 0.30.21
     optionalDependencies:
-      vite: 5.4.21
+      vite: 5.4.21(@types/node@22.19.19)
 
   '@vitest/pretty-format@2.1.9':
     dependencies:
@@ -1737,15 +1799,17 @@ snapshots:
 
   ulid@2.4.0: {}
 
+  undici-types@6.21.0: {}
+
   universalify@2.0.1: {}
 
-  vite-node@2.1.9:
+  vite-node@2.1.9(@types/node@22.19.19):
     dependencies:
       cac: 6.7.14
       debug: 4.4.3
       es-module-lexer: 1.7.0
       pathe: 1.1.2
-      vite: 5.4.21
+      vite: 5.4.21(@types/node@22.19.19)
     transitivePeerDependencies:
       - '@types/node'
       - less
@@ -1757,18 +1821,19 @@ snapshots:
       - supports-color
       - terser
 
-  vite@5.4.21:
+  vite@5.4.21(@types/node@22.19.19):
     dependencies:
       esbuild: 0.21.5
       postcss: 8.5.15
       rollup: 4.60.4
     optionalDependencies:
+      '@types/node': 22.19.19
       fsevents: 2.3.3
 
-  vitest@2.1.9(jsdom@25.0.1):
+  vitest@2.1.9(@types/node@22.19.19)(jsdom@25.0.1):
     dependencies:
       '@vitest/expect': 2.1.9
-      '@vitest/mocker': 2.1.9(vite@5.4.21)
+      '@vitest/mocker': 2.1.9(vite@5.4.21(@types/node@22.19.19))
       '@vitest/pretty-format': 2.1.9
       '@vitest/runner': 2.1.9
       '@vitest/snapshot': 2.1.9
@@ -1784,10 +1849,11 @@ snapshots:
       tinyexec: 0.3.2
       tinypool: 1.1.1
       tinyrainbow: 1.2.0
-      vite: 5.4.21
-      vite-node: 2.1.9
+      vite: 5.4.21(@types/node@22.19.19)
+      vite-node: 2.1.9(@types/node@22.19.19)
       why-is-node-running: 2.3.0
     optionalDependencies:
+      '@types/node': 22.19.19
       jsdom: 25.0.1
     transitivePeerDependencies:
       - less
@@ -1804,6 +1870,8 @@ snapshots:
     dependencies:
       xml-name-validator: 5.0.0
 
+  webextension-polyfill@0.12.0: {}
+
   webidl-conversions@7.0.0: {}
 
   whatwg-encoding@3.1.1: