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’s a separate collection of files and directories that’s only used by themes running on Vista. Elsewhere, Browser is the most important directory for customisation because it includes the most obvious graphics and some of the CSS. The Global directory includes CSS files that define the layout of the different browser windows, and Mozapps and Help define the look and feel of the Help dialog and the browser extras, including the Extension Manager and Update Wizard. Customised themes also contain extra files, including ‘preview.png’ – a preview of the skin.