Adblock Plus with the Filterset.G filter list (two separate extensions) - gets rid of ads while browsing
RefControl - removes the "referer" tag from the page request your browser sends to a web site
Redirect Remover - highlights links that redirect through another site and gives you the option to go to a "cleaned" link directly to the target
Cookie Button in the status bar - this combined with making Firefox ask you about cookies lets you control site cookies and third party cookies on a per-site basis
McAfee SiteAdvisor - adds an icon to display potentially dangerous or spammy sites in Google searches and in the status bar
Tab History - links opened in a new tab retain their history - i.e. if I open an a.net thread in a new tab, that tab has a back button and I can go back to the forum index.
Session Saver - when you open Firefox, all the tabs and pages you had open when you last closed it are reopened, including each tab's history.
Duplicate Tab - duplicate a tab in a new window - works with Tab History so the duplicate tab has the same history as the original
Download Statusbar - gets rid of the stupid "Downloads" window and shows downloads in a mini-progress bar on the status bar. Lots of options.
Fasterfox - options to speed up browsing
Tabbrowser preferences - because tabs are awesome and more tab features are even better
Tab - there is exactly one site I use that doesn't work well with Firefox, and this opens it in a new tab with the IE
Bugmenot - right-click on a login page to automatically fill in a Bugmenot signin from the context menu.
Dictionary Tooltip - double-click a word to open up a mini-window with definitions from a choice of about a dozen different online dictionaries.
Right-Click-Link - for fools who can't be bothered to insert URL tags, this lets you highlight a plain-text link and right-click to open it in a new tab
SlimSearch - right-clicking on a word to search for it gives you a new context menu to directly search Wikipedia, IMDB, and a bunch of other sites
As you can see, I love my extensions. I'll probably pick up some more if I see some good ones on this thread.