I try to keep add-ons at an absolute minimum. I may be losing out on some of the niftier features, but my time is too valuable for me to try out add-ons just to discover they were written in five minutes by some beginner /-:
Open in Browser - add "open in browser" to that pesky "download" or "open with selected application" dialog. Managed to get this to work in Firefox 3 with a bit of patchwork, although I think some of the recent FF3 upgrades broke that again.
It's All Text - add an Edit button which invokes your favorite editor (Emacs, natch) to edit text instead of those nasty edit boxes which are always too small and don't do anything useful or fun when you press Esc Meta Alt Control Shift
Restart Firefox - incredibly lame that this is missing from Firefox itself. Another one I managed to hand-patch in order to make it work with FF3, but again, alas, looks like I will have to do it again because it's missing from my menu once more.
A couple of runners-up which I use intermittently:
NoScript - a nice safety net, especially if you are confined to a Windows box on a gloomy, desperate day. Gets in the way sometimes but hey, that's security.
Linkwad - pretty lame, but I have not yet found a session manager which would allow me to save sets of tabs in a convenient form, and access them from anywhere. I really need to find something better. I'm a certified tab-aholic so my needs are probably a bit out of the ordinary. Any suggestions for what to try next?