1.) Del.icio.us Extension - this is an extension you can install to give you easy access to your online bookmarks via del.icio.us. Very cool and it makes it so simple to tag a page (control+shift+/) and view your bookmarks with the del.icio.us icon button. Check it out.
2.) Firefox Showcase - this extension lets your add the functionality of IE 7’s awesome “Quick Tabs” feature. Showcase allows you to view all of your open tabs in a single window and allows you to then choose the tab you’d like to focus on. Very cool feature from IE 7 now easily available for Firefox (which is why extensions rock!).
3.) FireFTP - this allows you to connect to various FTP sites with an awesome and simple interface within Firefox. Check it out!
4.) Greasemonkey - this allows you to customize the web in ways you might not be able to imagine, highly recommended.
5.) FavLoc - allows you to set favorite download folders so you can easily download files into different directories!
6.) DownThemAll! - installs a much needed download manager.
All of the Firefox Recommended Add-ons are awesome as well and I suggest you check them out to add much needed functionality to Firefox. I don’t go overboard, for example right now I only have Del.icio.us and Showcase installed because I like to keep a few, stable extensions installed for stability and simplicity. I also just keep the classic Firefox 2 theme because most other themes are not good enough for me 
Thank You