I tend to be in favor of less add-ons than more. The main ones I use are:
- Lastpass - An online password key manager. Also available for Chrome, etc.
- Xmarks (Foxmarks) - Bookmarks syncing. Also availbe for Chrome.
- All-in-one Sidebar - Pulls the extensions management into a nice sidebar and allows it to be easily accessed and modified.
- Omnibar - Integrates the search bar into the address bar a la Chrome.
- Menu Editor - Allows me to cut out alot of the crap in the menu bar that is useless if you know the shortcuts for what you need.
Other than that, I find most other things to be more obtrusive and distracting than useful. Admittedly I might want to run NoScript, but that takes a bit of effort to configure.
Otherwise, CoolIris if nothing else is interesting. It basically presents a wall of content (images and video) that you can zoom into and watch/look at.
Now if/when I work on a website (eeh... :() I will/would likely deploy a separate build of Firefox with many of the webmaster extensions added, such as Firebug, FireFTP, CSSViewer, ColorZilla, etc.