Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Post a screenshot of what you are seeing on YouTube.

I tried safe mode. It doesn't help. The screen of YouTube page in normal mode https://photos.google.com/search/_tra_/photo/AF1QipN5JTdCgK8pNiJHog_NjRiSPTmjbv-wzHhe1VY safe mode https://photos.google.com/search/_tra_/photo/AF1QipMsRXGu3Sq9cP1Fm1FWC5h8Ow6MVXbtL_xgwuU

Your links are not working.

https://photos.app.goo.gl/RodCQ17aRtHfUFut5 https://photos.app.goo.gl/N9tnn7XzMmbvCQRV6

sorry, I can't upload images directly via form at mozilla_org - images aren't uploading

Have you tried Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

What security software are you running?

I had switched off all components of Dr.Web, I had launched safe mode (Shift+double click), the playback didn't started. The problem appeared after updating the version of Mozilla (three versions before - not the current 96, but 58.**) Until that moment, there were no conflicts between the browser and the antivirus program.

Dr. Web... sounds funny, but with Dr. Web off did it work?

No, it didn't. In truth, DrWeb is not completely disabled - I need to completely delete and clean the registry in order to completely stop it. But if the problem is in the antivirus program, then I will have to switch to another browser. This is a corporate program and without it we cannot work on the network.

At home or at work? Are you on VPN?

This is laptop. I use it both at office and at home. Without VPN

Did you ever try Windows in Safe Mode?

with Win7 yes, but not with 10

Off you go then...