I am linking directly to the developers since, for some reason, direct links to Mozilla’s extensions causes Movable Type to vomit:

Autofill - saves LOTS of time
BugMeNot - Bypass compulsory web registration (i.e., NY Times registration just to read an article, etc.)
Colorful Tabs - helps differentiate tabs nicely
Copy Plain Text
Del.icio.us
Flat Bookmark Editing
ForecastFox
Foxmarks - saved my ass when my computer went down and took Firefox + all my bookmarks with it
IE View Lite - for those pages that require IE, this makes it easy to switch over
Morning Coffee - since I can’t have the real thing anymore…opens your favorite pages with one click
Video Downloader - for getting those YouTube videos and stuff
Google Notebook - great for storing little snippets here and there
Adblock Plus - because we’re all sick of hearing those obnoxious Flash ads
Session Manager - sooner or later Firefox starts hogging a lot of memory, and maybe you have a lot of tabs and unfinished business open. This is your ticket. Just save your session and restart
Oboe - for doing stuff with my MP3tunes.com account
Update: Better Gmail - does all this stuff