You are correct that not every function of the old menu bar is in the new Firefox menu. That's why we left the ability to switch back and forth. If you only need the old menu temporarily, hold down the "Alt" key to toggle it.

I don't know how to activate the menu bar.With the "Alt" key doesn't work and I don't know what to do.I want it temporary.

Some menu entries in the main menu bar are hidden if you use the mouse and only appear if you use the keyboard to open the menu (Bug 626825).
You can see the difference if you use Alt+F to open the File menu or other menus like the Edit menu (Alt+E) and Bookmarks menu (Alt+B) and compare that to what you see if you use the mouse to open the menu after you have made the menu bar visible by tapping Alt or by pressing F10.

Press the F10 key or tap the Alt key to bring up the "Menu Bar" temporarily if the menu bar is hidden.

Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).

• https://support.mozilla.org/kb/how-to-use-full-screen

See also:

• [/questions/799856#answer-155765]