I tried to put these in order of most useful or most used by me.

Add Bookmark Here – This is a great way to add bookmarks in any folder quickly and easily.
DownThemAll! – Download any link or content fast and stay organized as you do so.
IE Tab – Quickly switch to the native Internet Explorer rendering engine while staying in the Firefox experience
Download Statusbar – Shows you download status, and allows you to quickly open files you’ve downloaded.
NoScript – stay safe on the net using this script blocker, allow all and black list, or disallow all and white list.
Adblock Plus – Blocks pesky ads and works really well with the Adblock Plus Filter.G Updater, also can whitelist sites to allow ads on certain sites, Please disable this for free sites you use, as that’s how they stay alive.
Customize Google – a way to change the behavior of many of googles services
Greasemonkey – a way to add scripts that change the behavior or experience of any given website
Tab Mix Plus – Adds more features to Firefox’s native tab experience, not as useful as Firefox improves
Google Browser Sync – Allows you to sync up bookmarks and other stuff from computer to computer, handy if you use bookmarks alot. I’m trying to switch to Foxmarks as it allows a more controlled experience. GBS always syncs, and has corrupted once, but that was cause I used it within Linux.
Clipmarks – A cool service to let you save any portion of any web page, and comment about it and share it.
Google Gears (Beta) – A new offline technology that seems to work decently I’m currently testing using it.
Google Notebook – similar to clipmarks, if you use Google Notebook then this is for you
Sage – an RSS reader that’s useful and good quality, I use Google Reader now though so I don’t really
DOM Inspector – A nice DOM tree viewer for developers, comes with Firefox
Web Developer – a great extension for anyone that develops for the web
Firebug – another developer extension, haven’t used javascript much yet so have only played around with it for fun.