Firefox menu bar is hidden by default. If you don’t like it, you can restore the menu again. The other option is to unhide the menu only when you need it with a press of a shortcut key.
Restore Firefox menu bar permanently
Click Firefox button > Options > Menu Bar
In the screenshot below you can see that the Firefox button is hidden, replaced by the Menu bar.
Restore Firefox menu bar temporarily with shortcut key
Just press the Alt button on your keyboard.
The menu bar is instantly displayed once you press the Alt key and you can still see the Firefox button on its place. Moreover, after you press the shortcut, you can instantly access the menu items simply by using your left/right/up/down arrow keyboard buttons.