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.
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
Lightbeam is a Firefox add-on that enables you to see the first and third party sites you interact with on the Web. Using interactive visualizations, Lightbeam shows you the relationships between these third parties and the sites you visit.
Watch videos anywhere in cinema mode without leaving the website you're on.
Status bar widgets and progress indicators for Firefox 4+
Displays tabs in tiled layouts in Firefox. Arrange tiles horizontally, vertically or in a grid. Resize tiles by dragging splitter bars. Tabs and links can be dragged to open in other tiles. Supports multiple tiled layouts. Works with tab groups.
Make any pages' search functionality available in the Search Bar (or "search box")...
The greatest user script engine on the Internet (a fork of Greasemonkey).
Customize menus : Helps removing, moving and renaming menus and menu items
Colorize important menu for ease of use! (use Style (CSS))
Change or disable any of used keyboard shortcuts
Support: Firefox, Thunderbird and SeaMonkey
Allows to customize or disable most of the shortcuts offered by Firefox. This add-on does *not* allow to add new shortcuts.
Australian English Dictionary for Mozilla applications.