• Menu Icons Plus
  Left click to install this extension, or right click and choose "Save Link As..." to download it.  Version 1.9 for Firefox 3.0-3.6.* (Windows/Linux)

Menu Icons Plus iconifies your Firefox menus and more!  It includes 3 different icons sets to choose from, along with the ability to import/export icon sets.  It also provides options for changing other aspects of the toolbars' and menus' appearance, like combining the Stop and Reload buttons on the navigation bar or hiding the bookmark button in the location bar.

Once it is installed, start Firefox and open the Add-ons list from the Tools menu.
Select Menu Icons Plus and click the Options button (or Preferences button in Linux).
Here you can change which icon set to use for the menu icons, and set different preferences for how the toolbars and menus appear.
Click the OK button (or Close button in Linux); if your changes don't take effect right away, restarting Firefox should do the trick.

For instructions on how to make your own icon set, see the custom icon set tutorial.

Disclaimer:  Some Menu Icons Plus options may be incompatible with Firefox themes that deviate a lot from the standard theme.  Also, Menu Icons Plus has some bugs yet to be worked out on Windows 7, and it has not been thoroughly tested on Windows Vista.

Screenshot 1: View menu with Crystal Project icons and glass selection


Screenshot 2: Tools menu with Silk icons and rectangle selection


Screenshot 3: Context menu with Tango icons in Ubuntu Linux


Screenshot 4: Icon set preferences


Screenshot 5: Toolbar options-Windows


Screenshot 6: Menu options-Windows