Clipmarks
Ever wish you could "clip" a piece of a Web page the way you'd tear an article from a magazine? Clipmarks lets you do exactly that, then tag, store, organize, and share your clips. The site's simple toolbar is available for both Firefox and Internet Explorer.
DownThemAll 1.0
This download manager lets you speed up, queue, and batch downloads from the Web based on your previously defined patterns.
Foxmarks Bookmark Synchronizer 1.0.1
Foxmarks synchs your bookmarks among different PCs and operating systems, making it perfect for getting your office and home bookmark libraries in harmony.
FoxyTunes 2.96
FoxyTunes can control any of over 30 media players (including iTunes, Musicmatch Jukebox, and Windows Media Player) straight from the browser window. Artist and track information, play controls, and a slider to adjust volume are presented discreetly in your browser.

Gmail Manager 0.5.4
This extension that lets you manage multiple Gmail accounts and receive new mail announcements as a pop-up on your status bar, eliminating the need to keep Gmail open in a separate window.

Googlepedia 0.5.1
Googlepedia shows you the most relevant Wikipedia article alongside your Google search results.

Greasemonkey
Greasemonkey lets you customize the way Web pages look and work. There are hundreds of scripts available for free, allowing for a huge variety of customization.

Reload Every
This extension allows you to program Firefox to refresh all open tabs, or one tab, after a set amount of time. It's especially useful for monitoring very active forums.

Session Manager
Session Manager lets you save the layout and addresses of all your tabs when you shut down Firefox, allowing you to keep that same layout when you open the browser up again.

Tree Style Tab 0.6.2
As the name suggests, this extension allows you to have a tree-style tab bar, like the folder-view tree that's used in Windows Explorer.

tree tabs? I think you'd be better off sticking to bookmarks if you need a tree.