• Menu Icons Plus
  Left click to install this extension, or right click and choose "Save Link As..." to download it.  Version 3.2 for Firefox 4-56

Menu Icons Plus iconifies your Firefox menus and more!  It includes several 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 menus' appearance.

Once it's installed, start Firefox and open the Add-ons Manager from the Firefox menu or 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 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.

Screenshot 1: App menu with Fugue icons on Windows 7


Screenshot 2: File menu with Crystal Project icons on Windows 7


Screenshot 3: Context menu with Tango icons in Ubuntu Linux


Screenshot 4: Icon set options


Screenshot 5: Menu options