Is the problem that Firefox keeps changing to the compact two-column menu accessed through the orange Firefox button, or does the menu bar disappear without being replaced by the orange Firefox button?

Sometimes the file that stores your toolbar customizations (and various other UI settings) becomes corrupted and causes random behavior. You can rename the file to restore the default toolbar layout and customize from scratch. Here's how.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leaving that window open, switch over to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Restart Firefox. Hopefully now your toolbars will be stable!

Whoops, if you can't display the menu bar by tapping the Alt key or F10 to get the Help menu, you can open that Troubleshooting page by typing or pasting about:support in the address bar and pressing Enter.

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

If you are in full screen mode then hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear.
Click the Maximize button (top right corner of the Navigation Toolbar) to leave full screen mode or right-click empty space on a toolbar and choose "Exit Full Screen Mode" or press the F11 key.