Why do you need more than one content blocker? You may have to disable them for youtube.

If you have strict or a similar setting for Enhanced Tracking Protection, that could be the cause of your issue. You could try switching it off. While on the site, click the shield icon at the far left of the address bar and use the button to switch off ETP.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

it turned out that ironically i cannot view youtube videos while the extention 'youtube experience' is active...

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