Most of the users have seen Microsoft Windows 7 UI and liked a lot. If you want to make your firebox look like windows 7 then here is windows 7 theme for firebox designed by one of deviantart member.
In order to use Windows 7 Theme for Firebox you need to install Glasser Firefox add-on to emulate Aero Glass effect. Just download windows 7 theme for firefox and copy the userChrome.css in this archive to %appdata%/Mozilla/Firefox/Profiles/yourProfile/chrome and reopen Firefox.
Screenshot of Windows 7 Theme for Firefox 3.5/3.1 Beta