Adblock Plus
BlockSite
Cookie Monster (never use it though)
CustomizeGoogle
Firebug
Firecookie
Firefinder (haven't used this either)
FireRainbow
FlashGot
FoxyProxy
GreaseMonkey
ImageBot (buggy, don't use it often)
Personal Menu
RefControl
Refresh Blocker
Resizable Textarea
Session Manager
SkipScreen (find it doesn't help much)
Tab Mix Plus
Ubiquity (haven't used this either)
User Agent Switch (rarely use it, but I guess handy to have around if necessary)