Original screenshots didn't post. Posting now.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues

Hi FredMcD!

Thanks for your reply! Since the problem seems to come and go randomly, I will just have to run it a while with all disabled and see. It may take a bit of time, but I really plan to be able to come back and finish this off/close the matter. I will let you know soon.

Thanks again.

Hi,

Just in case none of the above make this nuisance go away for good, you could give this a try :

Type in the address bar about:config (press Enter) (promise to be careful, if asked)

Type in the search bar and look for the preference :

media.wmf.dxva.d3d11.enabled

and set it's value to false

Then close and restart Firefox, in order for the change to take effect.

Should that makes no difference either, then change the value back to 'true'.