Since the late 70s and well into the 80s currently version 88 of Firefox I had a problem with most videos displaying the following message:
"This video This video cannot be played because of a technical error.(Error Code: 200001)"

I have no proxy set and all drivers are current.
Researching I found many having same or similar problems but nobody having a solution.
That was until I came across a locked and archived forum thread: https://www.reddit.com/r/techsupport/comments/aday21/video_error/

The last reply, by "SlickStretch" was the correct answer and solution.

"solved by disabling the extension: 'Disable HTML5 Autoplay.' Try disabling/removing this extension or any similar ones."