Firefox uses a stylesheet file called userChrome.css to load customized design settings for the program interface. Modifying this file will give you a large variety of possible interface customizations. Firefox installs come with a same file which you can build from. Go to Documents and Settings > USERNAME > Application Data > Mozilla > Firefox > Profiles > XXXX.default > Chrome folder, usually on the same hard drive partition your operating system is installed on. USERNAME will be your Windows username and XXXX will be a random character string which is different for every Firefox installation. In that folder you should see a userChrome-example.css file. Rename this to userChrome.css. Then you can perform stylesheet modifations to customize the Firefox GUI. You need to reload the Firefox program for any changes to take effect.

You can go to www.mozilla.org/unix/customizing.html for sample customizations.