AdBlock + Adblock Filterset.Gto get rid of annoying ads on web pages (yes, I know they finance themselves through ads, and I do allow those that I want to support)
del.icio.us Bookmarksnot only are my bookmarks available from everywhere - this toll also organizes them by tags and allows for searching
Firebugessential for developers
FlashGot + Free Download Manager plugin (FDM)a must-have for downloading stuff
FoxyProxyallows you to use multiple proxies dependant on the URL
IE Tab
sadly, this is necessary because there are still web sites out there that render only in IE
NoScriptprotection against malware
Redirect Remover
for better privacy - I hate it when search results tell the search engine, which links I followed
Session ManagerSaves your browsing sessions, in particular when FF crashes, but also whenever you wish to save so you can return later, and of course when you close FF, so you do not need to fear loosing information by closing FF any longer.
Temporary Inboxyou have to enter your email address every so often. This allows you to use a temporary email address and receive emails for verification too
Tree Style Tabshows tabs as a tree, opens new windows from one tab as a sub-tab, and allows you to close a whole branch
Web Developeressential for developers
YSlownice-to-have for developers
I also use these from time to time (nice to have): Download Statusbar, Forecastfox, Map+, Media Pirate, PDF Download, Usewr Agent Switcher