You can tap the Alt key or press the F10 key to show the hidden Menu Bar temporarily

Open the Customize window and set what toolbars (Show/Hide Toolbars) and toolbar items to display.

• Right-click empty toolbar area -> Customize
• "3-bar" menu button -> Customize
• View -> Toolbars
  *you can tap the Alt key or press the F10 key to show the hidden Menu Bar temporarily

Make sure you do not run Firefox in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

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

When you are in Full Screen mode, hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear. You can use one of these to leave Full Screen mode:

• press the F11 key
• click the Maximize button at the right end of the Tab bar
• click the Full Screen button in the "3-bar" Firefox menu button drop-down list
• right-click empty space on a toolbar and click "Exit Full Screen Mode"

Which Firefox menu is missing?

If you meant the classic menu bar (File, Edit, View, History, Bookmarks, Tools, Help), then the steps in cor-el's reply should help you restore that.

If you meant the toolbar button with three horizontal lines, you may need to use the "Restore Defaults" button on the customize tab. See: Customize Firefox controls, buttons and toolbars.

If you meant the orange button that was present in Firefox 4-28 that said "Firefox", if you have seen that in recent, it probably was due to using the Classic Theme Restorer extension. Does that sound familiar?