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I need a refresh or restore or something like a reinstall to fix the overlarge display

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

• Cmd+Shift+f (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
  • click the View menu > Full Screen

Does that work?