I just cleaned up my Firefox setup.
I'd a lot of problems with it (two windows appearing when openning a link from RSS Bandit, frex) and was generally displeased with the way it was operating.
I uninstalled it, removed the %APPDATA%\FireFox and %APPDATA%\Mozilla {Lost my bookmarks in the process, because I forgot to backup, but I rarely use bookmarks anyway.}

I then installed Firefox 1.0 Preview Release and the following extentions {In order of usefulness}:

• Googlebar - Nothing more needs to be said.
• Tabbrowser Extensions - Very useful, lots of options. But what buys me is that it removes the "Open Link in new window" when you right click a link. My default behaviour is to right click and choose "Open link in new tab", so it's very conveniant to me to have it as the first item on the context menu. Lots of other, very cool options.
• All-in-One Gestures Extension - Allows simple {Left, Right, Up, Down} gestures, with a UI that I can understand. I used RadialContext for a time, and that was nice, but I had a lot of mistakes, because it's very small interface.
• Adblock - My eyes usually remove ads because my mind gets them, but this is nice to have.
• Linkification - Scan the page for plain text URL and turn them into links, a real time saver.
• Google pagerank - Nice to have, currently this site doesn't even appear on Google.

One thing that sucks in Firefox is their approach on installing stuff. It {correctly} blocks anything that is not defined {Basically, anything not on update.mozilla.org} but it gives you no option to temporarily allows installations. You've to allow the site to install stuff, install it, and then remove the site manually.

IE's approach since SP2 is much more sensible, "Allow this time only."