YouTube now defaults to AV1. The extension was last updated 6 years ago, so it still only blocks VP8/VP9.

EDIT: After changing this setting to false in about:config it works: media.av1.enabled

THANK YOUUUU

Fixed my lag

This add-on fixed my problem with YouTube immediately, I have a high end PC and it was freezing itself at 2x 1080p60. Thank you guys!

Not working anymore - I still have av1.

A must have for old laptops with no VP9 hardware decoding support. Just make sure that you can only watch videos up to 1080p.

Great improvement, in terms of keeping my laptop much cooler. Great extension. 👍

Finally videos don't get pale in full screen. Thanks you!

Installing this fixed hardware video decoding. Now Firefox uses much less CPU even when I watch 4K videos with VP9. Task manager shows HW video decoding works. It was zero before.

Still seems to work, finally no more choppy and painfully slow and in low resulution loading YouTube videos in Firefox, and no need to use YouTube in Chrome as a "workaround".