As a developer of a Firefox Toolbar, I know that Firefox can be hacked easily as it uses a lot of JAR files that are actually pretty much ZIP files you can unzip.
Anyways, the guys over at TechRadar go into depth of how to hack the Firefox theme to your liking so check it out:
Themes are supplied as ‘.jar’ files, which are really renamed zip files. You’ll find several directories inside: Browser, Communicator, Global, Help and Mozapps. There may also be a directory called Aero that duplicates some of these; effectively it