My Favorite Firefox Add-ons
Foxmarks Bookmark Synchronizer
If you use Firefox on more than one computer, you’ll want Foxmarks. Install Foxmarks on each computer, and it will work silently in the background to keep your bookmarks synchronized. You can also log in to my.foxmarks.com to manage your bookmarks from any computer.
StumbleUpon
StumbleUpon discovers web sites based on your interests, learns what you like and brings you more.
With StumbleUpon you can also connect with friends and share your discoveries, meet people that have similar interests, and check out what other people are discovering.
https://addons.mozilla.org/en-US/firefox/addon/138
Greasemonkey
Allows you to customize the way a webpage displays using small bits of JavaScript.
Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.
You can write your own scripts, too. Check out http://wiki.greasespot.net/ to get started.