Adblock Plus - Gets rid of all ads, with subscriptions to all official lists
AutoHideStatusBar- Hides the StatusBar when nothing's loading or I'm not hovering over a link.
BBcode- Quick little helper for posting in forums without having to learn all tag by hand
Chromedit - Brings up a nice little editor for editing how the browser looks, how webpages behave etc. using Firefox built-in CSS-thingie.(see below)
CustomizeGoogle - Adds plenty nice tweaks to google, making it much easier to use
FEBE - Backup-utillity for Firefox, i run this maybe once a month. Good to have when re-installing Firefox
Flashgot - "DOwnload manager"-manager. Supports all and any flavor of download manager.
IETab - Switches to/brings up a tab rinning the IE-engine, for those web-pages that are stuck in the 90's
MR Tech Disable XPI Install Delay - Gets rid if the delay when installing Extensions.
Nightly Tester Tools - Disables the compatibility-check when installing extensions in versions of Firefox the extension was never meant for.
Norwegian Dictionary - Obvious really :P
ReloadEvery - Reloads pages at a set interval.
Restart Firefox - Adds a menu-item under file that restarts Firefox.
Tiny Menu - Compacts the 'File view...' into one menu-item and puts them all as sub-menus.
Titlebar Tweaks. Gets rid of the "Mozilla Firefox"-tagline in the titlebar.