Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
View and manage downloads from a tidy statusbar - without the download window getting in the way of your web browsing.
Thanks Tekzilla for your awesome review! Click here to see their video: http://youtu.be/BUDqWrJ-Vao
Automatically find user scripts on Userscripts.org (requires Greasemonkey or Scriptish)
TooManyTabs allows you to store as many tabs as you like by adding extra rows in the Firefox! It saves your browser's space and memory as idle tabs are put aside. The extra rows also help to better prioritize and visualize your tabs.
Adds advanced search capabilities to the Firefox address bar. Supercharge the "Awesome Bar" with instant search results that appear as you type!
Note from Mozilla: This add-on has been discontinued. Try Roomy Bookmarks Toolbar instead.
Extends the Location Bar with set of commands to (Shorten URL's,Copy URL,Search site,Go up, pNg, Tag pages easily ,Navigate through sequential URL's,View page cached version,Surf anonymously using online phproxy servers, Twitter)