Note: this extension conflicts with my another extension Packed Menu 2 Nav, which puts the button "M" on the Navigation Toolbar. If you have installed Packed Menu 2 Nav, you must uninstall it first before you install this extension.
This extension adds a button "M" to the menu bar. And the original main menus are hidden. When you click the "M" button or press Alt-M, the original main menus are shown under the "M" button.
This extension has nearly no impact to the Firefox's performance. All it does is just adding a menu "M" to the tool-bar and moving the original "main menu bar" to the popup of the menu "M".
You can login to view the source code.This extension does not support Mac.