Could you make sure that Firefox is not in its "full screen" view? On Windows, you should be able to exit Full Screen view using either:

• F11 key (this switches full screen on and off)

• slide the mouse pointer up to the top of the screen, try a couple points if necessary, and the toolbar area should roll down, then either:
  • click the "3-bar" menu button and Full Screen
  • right-click > Exit Full Screen

Does that work?

Thanks for the suggestion. The problem was that full screen was not on, and I stumbled upon this just before I opened my e-mail and found your reply to my problem: I clicked the Open Menu icon, not knowing what it was, and clicked on Full Screen, just to see what would happen. And the browser screen returned to normal! Thanks again.