On Mac the menu bar is under control of the Mac OS and you can't affect this via Firefox settings or possibly via userChrome.css. I think that on Mac the menu bar will always be visible under normal conditions unless you enter Full Screen mode (not sure about pop-up windows).

Do you know what actions are causing the menu bar to get hidden ?

When I am in full screen, the menu bar pops up. So it is an OS issues? ok. I thought it was firefox. Thanks. But now I just need to know how to have the menu bar stop from popping up while in full screen

Are we all talking about the same menu bar: the one at the very top of the screen shared by all applications?

I think full screen should hide that, but different methods may give different results:

• using a full screen button on a video player
• using Command+Shift+F to toggle "browser full screen" on/off (or the equivalent command on View menu, if it's there on Mac)
• clicking the green button on the window frame

Okay, now I am reading your question a different way. Is the problem only when you are in full screen and your mouse pointer gets to the top of the screen -- the menu bar and/or other bars roll down at that point?