Here is mine:
SEO for Firefox Extension: Free SEO Toolbar Firefox Browser Plugin
What is yours?
Tab Mix Plus is a great one. Firefox would be barely usable for me without the vastly improved tab management it provides.
Submit Your Webmaster Related Sites to the NB Directory
I swear, by my life and my love of it, that I will never live for the sake of another man, nor ask another man to live for mine.
Google Reader Notifier
Direct HTTP Headers
Undo Closed Tab Button
HunSpell French Dictionnary
Will.Spencer (9 February, 2009)
These are the ones I'm using atm:
Java Quick Starter (Standard on FF?)
Google Preview 3.17 (shows websites screens when surfing google)
The ones I mostly use is SearchStatus (on every site, I check most sites' alexa & PR)
This is it I think, might add some more as I see there are good ones in this thread