I just wanted to chime in with my favorites. Every time I install Firefox on one of my systems, I instantly install the following:

-DownThemAll (IMO, the best extension out there)
-AdBlock (of course)
-FoxMarks (use it for syncing bookmarks on my desktop and laptop computer)
-AutoHideStatusbar and Hide Menubar (I want to maximize the browsing space, especially on my laptop due to a lower resolution. I hide the status bar unless I hover over a link, and I can bring the menu bar up with the Alt key)