Show or Hide the Menu Bar in Firefox the Easy Way
Do you only need occasional “easy access” to the Menu Bar in Firefox but want it hidden the rest of the time? Then you can enjoy the best of both worlds with the Hide Menubar extension for Firefox.
Before
Here is our browser with the “Menu Bar” showing full-time. Nice, but what if we could have a “built-in” on/off switch for the “Menu Bar”?

After
Installing the extension is quick and easy…and there are no options for you to worry with. As soon as you restart Firefox the “Menu Bar” is already hidden as shown below (nice!).
So what do you do when you need that occasional access to the “Menu Bar”? Just hit the left or right “Alt Key”…it acts just like a toggle switch. The “Menu Bar’ will also auto-hide whenever you click inside the webpage or outside of the browser itself. Sweet and simple!

Conclusion
The Hide Menubar extension lets you enjoy having the Menu Bar there when you need but keeps it out of your way when you do not. Enjoy!
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I would instead use this add-on https://addons.mozilla.org/en-US/firefox/addon/4550 so the menu is available handy though it does not take up any space.
But if you want no access to the menu at all, then this is fine.
The Tiny Menu extension does something similar to the Compact Menu addon that Anonymous posted above, but also allows you to use an icon of your choice. Move the buttons and address/search bars into it, and you have a compact menu that’s always available.
why not just right-click on the menu-bar, and un-check “Menu Bar” in the pop-up menu? This way, you can access the menu-bar at any time by pressing the Alt-key…
The action can easily be reversed by right-clicking on any of the navigation-buttons, and re-checking it..
Marius from Norway