Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

I tried your solution - using safe mode and it didn't work - then realized I have this problem in all screens, not just on Firefox. I think it must be a Windows 10 issue so I'll check further with that program. Thanks for your efforts, I appreciate it!  :)

Did you try the system menu (Alt+Space)?

Try to rename (or delete) the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

cor-el said

Did you try the system menu (Alt+Space)? Try to rename (or delete) the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thank you for your answer - actually it didn't work but I wanted to end the thread so I clicked that this answer worked for me. I wasn't sure how to get my comment published.

My issue, I realized, wasn't a Firefox issue. In Windows 10 there's an option for laptop or tablet, and I had inadvertently chosen my laptop to be a tablet. That turned off lots of things that were no longer available to my laptop. When I found that solution in a Windows 10 forum and tried it, everything was restored. I appreciate the people who tried to help me on this support group.  :)