This was solved by creating a new profile as Cor-el suggested in the other identical question. That question was "archived" so I couldn't post there to let Cor-el and everybody else know that it was fixed!! The only way I knew how to do so was to ask the question again. Strange indeed. This forum works unlike any of the others I belong to- I must be missing something.

Anyway, creating a new profile solved the problem! I've re-installed all the add-ons I had in my old profile including Adblock Plus, have hardware acceleration on, have tracking protection on, and have a non-default theme installed. So my new profile looks just like my old one. Except the problem is gone. Thanks again Cor-el. Hope this helps others.