Most Used: – Any needs to have this extension.. fanfntastic!
Firebug – A web designer must, seriously slick css/xhtml editor.
Adblock – Destroyer of the web 2.0 revenue model!!
StumbleUpon – This is like internet on crack!
FireFTP – FTP in browser with drag & drop.. niceeee.
TabMixPlus – Really useful for messing around with tabs.

Used Occasionally:
Panic Button – For when the boss comes rolling round whilst you are writing a blog post.
ScribeFire – Blog posting in browser.. but most of the time i prefer using WordPress itself.
Bugmenot – For finding registration for sites you don’t want to register for.
Sharealicious – For all those social media junkies.. easy to share!