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.
A customizable mouse gestures extension which enables to execute various commands and user scripts with six types of gestures.
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
Customize application menus:
Rearrange or remove menuitems from the main context menu (right-click menu) and main menubar (File Edit View etc.)
Note from Mozilla: This add-on has been discontinued. Try Colorlicious instead.
Change Firefox to Any Color and create your own personalized theme.
Fission combines address bar and progress bar (Safari style). This makes the progress bar more visible and allows for a nice visual effect.
Provides the option to copy selection as plain text or html...