I tried turning off all the add ons. Still some videos on Youtube will not play, just a loading circle. Most videos do play though.

Do you by chance have a KN or N edition of Windows 11 ?

https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

Are there media related messages in the Web Console ?

• "3-bar" menu button -> Web Developer or Tools -> Browser Tools -> Web Developer Tools
• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

Hi mervcol,

It seems like you're hit by bug1782322. Can you confirm the issue does persists when you run Firefox in Troubleshooting Mode?

You can try to adjust the video quality (away from auto) or un-maximize and reload the window to force a re-render to see if that makes the video start.