I found if I disabled the drag / drop / upload extension that the issue went away -- though I had to disable it twice and restart twice for whatever reasons ... but now the issue is gone.
I did have to position my cursor over the left-top Firefox menu and then click Enter a few times to get the extensions / add-ons area open ... and then I had to do similar maneuvering when the area was open.
This issue can be caused by an extension that isn't working properly.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.