Get Stumble Upon. It's wierd it's cool. And I discovered so many cool websites that I would have never thought to look for otherwise. The one I've wasted the most time on so far is gizmag.com (just try to spend under 4 hours there
Lets see the other extensions I have are...
ForcastFox
Google Preview - gives me a pic of websites before I load them
Bugmenot - Form filler for sites requiring information. Really handy.
GoogleToolbar - Haven't used it as much as I thought I would. The un-official one is better
Enjoy Firefox!