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.
After the installation, the menu bar will show just one item and you will get a lot of space there. You can do a little customization on the tool-bar to move things to the menu bar and get rid of the navigation toolbar if you like a compact tool-bar.
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".
This extension can be downloaded and installed from Mozilla Firefox's official extension web site:
Because there is no impact on functions of the original main menu bar, you can still use the short-cut keys to access the submenus. For example, Alt-F will open the popup menu under "File". But the "main menu bar" itself will not show up -- This is not a bug.
Install another extension Autohide Tabbar together and you would get a neater interface.