AdBlock Plus
Self explanatory
Customize Google
Who doesn't want to?
Fasterfox
Have no idea if it actually does anything, but at least you think it's going faster
Firebug
Ultimately useful for debugging Javascript. Also very useful for other things like viewing request/response headers etc
FlashBlock
Helps block annoying flash ads, and flash content
Flashgot
Integrates with Flashget
FoxyProxy
Handy for, well, proxies...
ImageBot
Quick uploading to Imageshack
RefControl
For faking referrers to lame sites that disallow hotlinking
Resizable Textarea
Makes posting easier by allowing you to make those text boxes bigger
StumbleUpon
Kinda okay if you're really bored
TabMix Plus
Much nicer control over tabbing.
There's a few other ones I can't remember, but ^they would be the main ones.
Here was an older thread By the way:
http://endlessparadigm.com/forum/showthread.php?tid=235