Can check the sound mixer? https://www.thewindowsclub.com/the-new-windows-7-volume-mixer-control

I checked all audio settings prior to posting a request for help. This problem only occurs on Firefox, in Chrome everything is fine.

Show your volume mixer, post a screenshot showing Firefox. I'm not able to replicate the issue on YouTube. see screenshot What security software are you running? Have you tried Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search Did you clear your browser cache? https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox