Sounds like the site player is incompatible with something in the Browser itself. Link helps others see what is going on here.

Does it still happen in Troubleshoot Mode?

Tried Troubleshoot mode and no soap. Just happened with this recent update. I turned off enhancements and that didn't solve it either. Only on FreeVerse so far. Works fine for other streaming.

It is Prime on Amazon which has FreeVerse hooked it. Prime itself is posing no problem and everything worked until this last update. That is all I can tell you. I'm using Chrome now anyway and it doesn't seem to have a problem with it. I tried Edge too and no problem.

Does it still happen in a new profile? An easy way to get a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Nope, didn't work. I'm using Chrome for Freeverse. I appreciate all the help, but I want to watch them and to keep replaying the same movie may draw unwanted attention. As I see it FireFox changed something and I can't unchange it.

You can test older versions with mozregression to find exactly when the problem started.