Firefox, Thunderbird, and Mozilla Suite (aka Seamonkey) share something in common: the ability to control the appearance by creating/editing preference files that are not accessed by the Edit - Preferences controls.
I refer to the user profile in your ~/.mozilla/firefox (thunderbird, mozilla) profile (/home/<username>/.mozilla/firefox/<random string>.default/chrome). The files to create/edit are userChrome.css and userContent.css.
You may have empty example files there. You can find more detailed information (with several links to user contributed files that you can copy and edit for your own use).
Of course, it helps if you know something about css (cascading style sheets). But, they're easily researched via www.google.com/linux