These are the ones I like, by order of importance, best ones first:
Xmarks
let's you have all your bookmarks stored online, enabling synchronization between several computers (at home, work, etc), can also store passwords (I don't recommend it). It's the replacement for "google browser sync". With this one I stoped emailing links to check at home, now I just drag them to the bookmark toolbar.
All-in-One Gestures
Execute common browser commands using mouse gestures.
Tiny Menu
Replaces the standard menu bar with a tiny menu popup
Stylish
To change the style of any page. I use it to block pictures and adds, transforms pages to black background with white text, etc.. It helps to know CSS.
DownloadHelper
Download anything from a web page, including all those youtube videos
IE Tab
For those annoying sites that only work in IE, this reloads a page using an Internet Explorer engine, right on firefox