I found this page on mozilla's main site. In almost every installation of firefox, firefox installs in two places. One place is the normal place ("C:\Program Files\Mozilla Firefox" by default) and the other place is in the application data (C:\Documents and Settings\
username\Application Data\Mozilla\Firefox\Profiles\
randomNumbersAndLetters.default\chrome by default). If you go there you will find a file called userchrome-example.css or userchrome.css.
Userchrome modifies the visual components that firefox uses. userchrome-example is simply a backup copy in case you screw something up. if you either open up userchrome or copy and rename userchrome.css, you can use the keywords defined on
this page to modify your current skin. I've had a lot of fun making my own custom skin!