1.
NoScript - I honestly can't live without this one, and it has been able to save me from numerous spyware infections. This is also the reason that I won't switch over to Google Chrome, no matter how much faster it might be.
2.
Adblock Plus - Say bye bye to all those annoying Flash ads! No more punch George Bush for me.
3.
CookieSafe - Blacklist for known bad cookies, and much better control over how cookies are handled on my system.
4.
XMarks Bookmark and Password Sync - I wipe and reload my system much more often then most people do, so having a way to easily have my bookmarks and passwords reloaded after a new install of Windows is a must. Also works great for Firefox Portable for when I'm at school.
5.
WOT (Web of Trust) Safe Browsing Tool - Lets you know if a site you are visiting has a bad reputation or not, so you can stay clear of a possible infection.
6.
Lazarus: Form Recovery - Auto-saves all forms so that if accidentally navigate away from the page, you don't have to retype that long forum post in the Core i7 Overclocking Guide.
7.
Tab Mix Plus - Best tab control extension I've ever found. Great for making you open your homepage when opening a new tab, and controlling where pop-ups show up.
8.
Smart Booksmarks Bar - Hides the names of your bookmarks, and replaces them with the site Favicon. Also great when used in conjunction with Favicon Picker 3, so you can pick your own Favicons for your bookmarks.
9.
Download Statusbar - Takes away the annoying download window, and replaces it with a bar at the bottom of the window that's only visible after downloading something.
10.
OptimzeGoogle - Let's you clear out google's advertising from Gmail, youtube, and most all other google services; let's you switch to https for your gmail; and let's you permanently set your google settings (i.e. search filter, number of results per page, etc.)