AdBlock Plus - AdBlock Plus has changed my internet browsing - I carry Firefox on my USB key, so I rarely use a browser without it. I really don’t see many more adverts anymore. This is a MUST for all Firefox browsers. Keep it updated with the Adblock Filterset G. Updater.
Adblock Filterset G. Updater - As stated above, Adblock Filterset G. Updater keeps AdBlock Plus up to date with new websites and definitions, so you are always up to date with the newest advertising companies.
Better Gmail 2 - If you use Gmail, this extension is a must. You can apply themes, lose the Google Adsense to the side, remove the Invite box on the left, and much more. A great extension for all Gmail users.
DownThemAll! - Ever wanted to save all the Videos, MP3s, pictures etc on to your hard drive? This is the add-on for you.
Foxmarks Bookmark Synchronizer - This is essential if you use more than one computer - like most of us who work in an office do. With this add-in you can access your bookmarks anywhere after installing this great add-on.
Fullerscreen - Want to browse at full screen, but still have quick access to the address bar? This is the extension for you. Maximize your browsing potential!
Greasemonkey - Customize most popular pages on the internet such as Facebook, MySpace, iGoogle etc. by downloading this add-on and visiting UserScripts.org and downloading scripts to use with this extension.
StumbleUpon - You probably found this site using StumbleUpon. If you did not… and you don’t have StumbleUpon, go get it now. Browse your favorite genre’s of web pages taking recommendations from people with similar interests.
Tiny Menu - Finally, a great extension to maximize your Firefox browsing area - use tiny menu to make the menu… tiny. See below. A small picture, but you can see that I can fit everything: Menu, URL bar, Bookmarks, Search box, and my StumbleUpon toolbar all on one row.