Menu Icons Plus
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.How to use Menu Icons Plus
Once it's installed, start Firefox and open the Add-ons Manager from the Firefox menu or the Tools menu.
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.Screenshots
Screenshot 1: App menu with Fugue icons on Windows 7