I won't bother to add links because:
A. I'm lazy.
B. If you can't figure out how to Google it, you're better off without extensions.
- Foxmarks: backs up and synchronizes bookmarks across machines.
- Flashblock: blocks flash. Or does it flash blocks?
- Adblock Plus: is it a good idea to mention this on an ad-supported blog? Probably not.
- Beagle Indexer: indexes visited pages using Beagle Search.
- CustomizeGoogle: enhances Google and its search results.
- Fast Dial: visual bookmarking. May be rendered obsolete in Firefox 3.1
- Screen Grab!: saves an entire web-page as an image.
- StumbleUpon: social bookmarking
- Tab Kit: tab grouping, multi-rows, tree view, etc.
- Torbutton: Anonymizes connection using Tor.
- RefControl: spoof http referrers.
The list will probably be edited from time to time with no warning.