Is it possible that Firefox is in full screen mode? You can toggle between full screen and regular using the F11 key. You also should be able to access the minimize, restore, and maximize buttons if you hold your mouse pointer over the upper right of the window for a few moments.

Also, a few keyboard shortcuts that might help:

Ctrl+w closes the current tab (or current window if you're viewing the last tab in the window)

Alt+F4 exits most Windows applications

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

I found the problem. It was the theme "Walnut" that was causing the problem. I have been using this theme since 2013 with no problems.

I switched to another theme and it fixed the problem.

Thanks to all for your suggestions. I have been a fan of Mozilla since the early days at NCSA at the U of I.

John