You may have discovered this by now, but sliding the mouse pointer all the way to the top of the screen should make the window title bar reappear along with the minimize/maximize/close buttons. On Windows, you can right-click and a small menu appears that allows you to exit full screen mode. (On Mac, maybe it's a long press??)

There are two of us who use the Mac. We each have our own name and password. After my husband signed in and then logged off...I went on. Everything went back to normal. Maybe it was just a glitch. But I'm glad it's over! Thank you for the time you spent replying to my post.

On Mac you should be able to toggle full screen mode via command+Shift+F.

• https://support.mozilla.org/kb/how-to-use-full-screen