Adblock Plus is an absolute must-have. It’s better than Adblock because it automatically updates its blacklist so it stays on top of things better.
Tab Mix Plus enhances tabbed browsing far beyond what Firefox offers, even now with all the improvements of Firefox 3. The feature list is fairly long and exploring all the options would be an adventure but it has good default behavior.
Forecastfox brings the weather to your status bar, both current and forecast. It can even show you an animated radar map.
The Wizz RSS News Reader is the easy path to RSS junkie-hood. You can easily monitor hundreds of different feeds, which is nice because it comes with about that many pre-loaded, all nicely categorized for your obsessive convenience.
Greasemonkey is in a class by itself: an extension that lets you extend it, quickly and easily (no reloading the browser!). Greasemonkey extensions are called User Scripts, and there are a whole lot of them, many of them site-specific. If you get Greasemonkey, grab Platypus as well so you can create your own User Scripts just by pointing and clicking. Greasemonkey and Platypus let you rewrite your view of the Web.