I'm running kubuntu and was very annoyed by the default icon of firefox (it's just a pretty dark worldbowl and really not nice to look at). Changing the icons was easy, but after I replaced the icons in /usr/share/icons/ the icons of firefox window were still the default ones. I figured out, that there are additional icons in /usr/pixmaps and in /usr/lib/mozilla-firefox that had to be replaced to get a uniform looking firefox. And last but not least, due to ruffuseddy I was able to adjust firefox's about screen, too.
This is pretty much copying that has to be done after each update of firefox - which lately became very frequent... So I packed everything needed together and wrote a little bashscript to do the work. Since one need to replace files in /usr/share and /usr/lib the root password is needed for proper execution.
Credits for the firefox icon go to aax24vb (http://www.kde-look.org/content/show.php?content=14153).
To install just upack and run the crystalfox.sh script.
IMPORTANT: As I said I'm running Kubuntu and adjusted the paths according to my installation. If you're running a different system, you should check and adjust the paths BEFORE running the script.