Perhaps you haven’t noticed but one thing Firefox lacks is smoothness. Not in the “coolness” sense but in smooth animations when doing some tasks like adding or removing tabs, raising dialogs, or opening sidebars.
Firefox 4 is expected to bring animations later this year, and while some people may just consider it a computer power waste, if you are into eye candy, and smoothness, you may want to try Liquid Tabs which turns tab adding and closing into cooler, easier animations.
You can set the speed and whether you want to fade in or out at the same time, at least on Windows 7. On Ubuntu 10.04, it was unable to keep my custom settings.
You can get Liquid Tabs from Mozilla Add-ons.