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Does it still happen in Troubleshoot Mode?

Cannot test it in Troubleshoot mode as I require FF for daily activities. If there is a way to run simontonously a few FF instances then I can try to run them pararell. The problem did not occur prior 101 or 102 version of infamous FF giant youtube video playback issues.

You only need to do a quick test in Troubleshoot Mode. Open a site with a video, then manually hibernate the system and see if it works after resuming.

Does it still happen with hardware acceleration disabled?

No it doesn't work like this. Usually it passes a few days/a few hibernations to notice a such issue. It must be some kind of build up. I cannot use bare FF for a few days, it must be pararell but firefox won't let to do that.

I can test OpenGL to OFF and hardware acc OFF as well and observe the behaviour.

You only need to do a quick test in Troubleshoot Mode. Open a site with a video, then manually hibernate the system and see if it works after resuming. Does it still happen with hardware acceleration disabled?

I've been testing hardware acceleration and it seems to be it. How it is related to media playback I cannot tell. FF had constant issues with it afaik in the past though. I will have to think if it is better to turn in on and have better performance or off but working media playback.

You can disable hardware acceleration just for video by changing media.hardware-video-decoding.enabled to false in about:config then restarting the browser.