Fire.fm
Note: To those who live outside the US, UK, or Germany Last.fm is not free. The price is either $3 or €3 a month.
This is a must have for anyone who frequents Last.fm. It adds a toolbar that gives you access to last.fm's music library, allowing you to listen to music while browsing the internet. Very simple to use, but if you don't know how to get it working simply click on the flashing orange radio signal. If you're one of those minimalists who worries about loosing too much space to oversized toolbars and junk like that, this is about how large it is.
(Word of warning, this add-on can cause Youtube videos to glitch or act weird, this is usually resolved by refreshing the page)

FoxTab
Adds 3D tabs to your browser. At first this doesn't seem all that useful, but if you also use Tree Style Tabs and hate constantly opening and closing the tab bar, then this is a life saver.