== How to fiddle with Firefox .jar files relatively easily

A lot of Firefox's code is actually written in various interpreted
languages, especially JavaScript, and Firefox extensions are pretty
much all interpreted. This means that you can do a lot of Firefox and
extension hacking without ever having to rebuild from source; you just
zap the interpreted files in place in your installed copy.

Firefox doesn't leave all of these files sitting around for you to
edit; it bundles them up in various _.jar_ files (pretty much all in
the _chrome/_ subdirectory). Fortunately _.jar_ files are just ZIP
archives.  The easy way to modify files in them is:

# unzip the relevant jarfile somewhere (I usually use _chrome/_
  itself, but I live dangerously)
# edit the unpacked files to taste
# run '_zip -f .../<whatever>.jar_' in the root of where you unpacked
  the jarfile to 'freshen' it with your changes
# restart Firefox to test things.

(There are recipes for running Firefox from unpacked versions of the
_.jar_ files, but they require some black magic.)

(Figuring out which is the relevant jarfile (and the relevant source
files) is beyond the scope of this entry; I suggest unpacking
jarfiles and poking around. Also, see the Mozilla knowledge base at
http://kb.mozillazine.org/.)