I'm using Firefox on a desktop (OSX Catalina 10.15.7) with multiple monitors. If I go into full screen mode on any youtube video , e.g., Welcome to Firefox (https://www.youtube.com/watch?v=tRlmP6xWdw0), not only do I lose access to the menubar on the fullscreen monitor, I lose it on the other two monitors I have open Firefox windows. The browser.fullscreen.autohide setting does not make a difference. I would expect that the only monitor that the menubar is hidden on is the one displaying the fullscreen content. The attached Monitor1, Monitor2 images show the missing menubar on the respective monitors where a Firefox browser window is open.