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.
Theme Font & Size Changer lets you change the global font size and font family used for the user interface in Firefox. Design your browser with your favorite font.
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
Status bar widgets and progress indicators for Firefox 4+
Customize application menus:
Rearrange or remove menuitems from the main context menu (right-click menu) and main menubar (File Edit View etc.)
A remake of the MinimizeToTray-extension made to fit the Windows 7 taskbar. Instead of minimize the window to the tray it minimize it to the taskbar. It can minimize on startup, when the window is closed and/or when the esc key is pressed.
Bring your bookmarks into view with just a gesture. Move the sidebar across the screen. Open a different sidebar by just clicking its title. And more! Keeps your sidebar simple and quick to access!
Customizes the new tab page. Add more tiles. Add the launcher from Firefox start. Have a background image. Set new tile images and titles. See recently closed tabs. And more...
Get a faster browsing experience with SpeedFox. Customized new tab and shortcuts to your favorite websites. Open the SpeedFox Overlay on any website with CTRL+Q. No more misspellings in your URLs with the automatic URL fixer.