To build this, "make" should suffice.

When accepting a zebra file from the UPS web site, this plugin will
send that file to a locally connected label printer.  You must specify
the print command that will be used.  This is found near the end of
plugin.h.  You need to define the PRINTER token to the name of your
printer.  Or if needed, you can redefine the whole LPR_CALL to some
other print command if your system requires it.  Note that the code
tries to substitute the value of PRINTER into LPR_CALL via sprintf so
you must have a %s in LPR_CALL which will get substituted with the value 
of PRINTER.

After rebuilding, you will have to install this for your local
mozilla installation.  This can be done by on a per-user basis by
browsing the local file: index.html.

To install system-wide (for all users), copy this file:

  libupsprintplugin.so

into /usr/lib/mozilla.../plugins or whereever you install your
mozilla.