The Firefox screen has resized itself, and the Tab Bar has rolled up off the top edge of the screen.
This may or may not have something to do with my installing & running PC Decrapifier & CCleaner on my desktop PC (I believe it started happening before then today). Anyway, intermittently, the Firefox screen gets too tall to display all at once on my widescreen LCD monitor, thus causing the Tab Bar to disappear off the top of the screen. If I move my mouse to the top edge of the screen, the Tab Bar reappears, but then disappears again after a while. The Tab Bar has been displaying for the last several minutes now, but I haven't done anything to make it stay put. (I wonder if the Firefox screen is confused about whether I'm using a 4x3 or 16x9 monitor? I access the Internet & my home email from my work computer as well; the work computer has an older 4x3 (squarish) LCD monitor, and uses Windows XP (while my home computer uses Windows 7).)
Solução escolhida
Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).
If you are in full screen mode then hover the mouse to the top to make the Navigation Toolbar and Tab bar appear.
You can click the Maximize button at the top right to leave full screen mode or right click empty space on a toolbar and use "Exit Full Screen Mode" or press F11.
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Solução escolhida
Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).
If you are in full screen mode then hover the mouse to the top to make the Navigation Toolbar and Tab bar appear.
You can click the Maximize button at the top right to leave full screen mode or right click empty space on a toolbar and use "Exit Full Screen Mode" or press F11.