Fullscreen will remove menubar from all desktops
I'm running Mac OS Mojave and when I play a video say from Youtube on a monitor in full screen, when I visit a web page on a different monitor the menubar disappears until I click into the empty space where it is supposed to be. In addition the dock will not show up when you move the cursor over where the dock is supposed to be (it's set as autohide) until you can get the menubar to show up.
I'm not sure if this is a MacOS thing or a Firefox thing but it is kind of annoying.
I have menubar hiding off in settings->general and still non-fullscreen Firefox windows will hide the menubar if there is a fullscreen window in ANY of my desktops.