Menu Icons Plus iconifies your Firefox menus and more. It includes built-in 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. View the screenshots to see what other options are included.
How to use Menu Icons Plus:
Once it is installed, start Firefox and open the Add-ons Manager from the App menu or 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.
Some options may be incompatible with Firefox themes that deviate from the standard theme in significant ways.
-Menu Icons Plus is not compatible with the CuteMenus or CuteButtons add-ons.