Here's how you can add menu icons to your Firefox browser by downloading a free extension called Menu Icons Plus.
1. Go to the Menu Icons Plus page.
2. Click the Download Now button.
Here's a description of Menu Icons Plus:
"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."
For more, see the original article at the link below.
Improve Firefox with Menu Icons | Ghacks