Use Restore or Maximize in the right-click context menu of the Taskbar icon to set focus to the Firefox application if you do not see the Firefox window.
Open the system menu of that Firefox window via Alt+Space and see if you can move and resize that window (use the keyboard cursor keys).
If that works then first close all other open Firefox windows and then close Firefox via "File > Exit/Quit" to save that setting.

You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Restore is grayed out, but Maximize brought up the window (with a dozen tabs open) in fullscreen mode. Alt+space made it all go away, just disappear. Move and Size both bring up a 4-way arrow cursor which disappears when I hit any key, including keyboard arrows. It's as if the maximize button takes it from full screen to minimize from fullscreen.

Hello, Can you please provide us with the screenshot of your problem, we may better assist you with that. For creating screenshot of your problem please see this How do I create a screenshot of my problem?

For escaping from full screen you can press F11 or Fn+F11 according to your keyboard configuration.

Aman, thanks for shifting my focus. You didn't solve my problem but caused me to do so while playing with the Paint program and PrtScrs. OK, problem solved. It wasn't a true fullscreen, just a whole screen maximized, which I later found that I could shift, via the title bar, to find an edge. After resizing and closing that, then reopening it, Firefox was fixed. I hope someone finds the person causing that mischief and spanks them good. What could cause Windows or Firefox to do this, though?