Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

   Click the menu button New Fx Menu , click Help Help-29 and select Restart with Add-ons Disabled. 

If Firefox is not running, you can start Firefox in Safe Mode as follows:

   On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
   On Mac: Hold the option key while starting Firefox.
   On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
   (you may need to specify the Firefox installation path e.g. /usr/lib/firefox) 

When the Firefox Safe Mode window appears, select "Start in Safe Mode". If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Thanks for the reply. I tried safe mode and it did not help. The only thing that helped was setting the resolution 2 notches lower to 540. Not sure if the problem has something to do with the copy protection that eventive uses though the same platform was used in the spring with no problem of PC Firefox. My iMac with OSX 10.10 which is below their 10.12 minimum plays fine on Firefox 78.5.0esr.

Can you try a video that we call can access without a login. We need replicate the issue, but can't do it with the link you provided. Is the issue only with that one video? I see that the site has video from different sources, but so far all is well for me.