Pushing the meme along with Oliver Thylmann and following my last extension-tagged post is my sizable Firefox Add-ons list!
Web Developer — essential; I <3 realtime CSS editing
Firebug — essential; the only way to debug; also see FlashBug for watching your traces() and FirePHP for eliminating error_log()‘s
Mouse Gestures — Hold middle click and motion the mouse right to go back, left to go forward, down & left/right to switch tabs, etc. ( also check out the QuickSilver mouse gestures module)