cancel auto hide menu bar in multiple desktop - Mojave

Permanent menu is there when you do NOT select fullscreen - that is how the mac has worked since 1984.

Apple added fullscreen some years ago, but you don't have to use it (I don't, most of the time).

As dialabrain said, you can maximize the window (option-click the green button) so that the menu remains but the window otherwise fills the screen (my tabs, fortunately, are below the address bar :-))

This is called "fullscreen".

If you are in fullscreen, and go up to the menu, presumably you want to do something with it, and then let go.

Otherwise, you might as well exit fullscreen mode.

When NOT in fullscreen, nothing occupies the space of the menu bar, so there will be no overlap.

When in fullscreen, and you bring up the menu bar, it slides down and temporarily covers what is in that area - that is how fullscreen mode works. None of us can change that.

On another direction, my Firefox tabs are below the address bar... I don't remember what, if anything, I did, but I do think that "tabs on top" is downright ugly (sorry). You may want to consider changing that, as an alternative.

None are impossible to operate in Full Screen. You just move the cursor to the top of the screen and the menu bar appears. You can also maximize windows instead of running in Full Screen. That way you will always see the menu bar.

If none of that is to your liking you can let Apple know… Feedback - macOS - Apple

FWIW, Full Screen was available long before Mozilla moved the Firefox tabs so perhaps it is they that should have thought of it.