Hmm, I think Mac OS manages the menu bar. I'm not actually aware of a way to (re)move it.

Full screen might hide it. Command+Shift+F should switch in and out of browser full screen.

Thank you for replying. My problem is that I DON'T have the Menu Bar at the top left of the Firefox window when I open Firefox. I have tried to find any option to activate it but haven't so far. The Menu Bar shows in my Firefox on my Windows 11 Pro laptop. And I can select/unselect it. But I cannot find an option to turn it back on again on my Mac Ventura. Any more suggestions?

Full Screen was my only thought about why it might not be visible on Mac. Hopefully an actual Mac user can chime in.