Got this?
Then go to Options (hamburger icon at far upper right side or ALT key-> Tools menu -> Options). Scroll down in General (which should be the default view) to Performance and uncheck Use Recommended Performance Settings. That will reveal Use Hardware Acceleration When Available, which will probably be checked. Uncheck it.
I now have normal YouTube playback again. Thanks to philipp_sumo on Reddit who gave me a trail to follow for this fix.
This bug seems prevalent in AMD graphics. Updating the driver didn’t help me. The above did.
Previously here:
A more specific solution which will have less impact on performance is to set media.wmf.use-nv12-format=false in about:config
Thanks for the tip. It can help others. Right now, YouTube plays better than it ever has, so I’ll keep mine as is.