You could try a plugin which forces h264 Youtube. Just about anything made in the last decade has mature hardware support for decoding h264, but not necessarily for VP8/9. This may be what's going on here.
Re:Firefox 70
by nathancahill on Tuesday October 22, 2019 @12:25PM
(#21325537)
The addon I use is: https://github.com/erkserkserks/h264ify
See https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...
More work is planned to reduce the energy usage for scrolling and full screen video. Though I guess for your example you don't watch things fullscreen.
Re:Firefox 70
by bluegreyred on Tuesday October 22, 2019 @11:18AM
(#21324601)
I just checked with the Intel Power Gadget on a 2015 MBP 10.14.6, iGPU only, same video at 1080p, package power consumption (CPU + iGPU):
~6,4W average in Safari 13.0.3
~8,6W average in Firefox 70
Safari 13.0.2 versus Firefox 70.0 versus Chrome 77.0
Playing the same YouTube video at the same quality (1080p) and watching Energy Impact in Activity Monitor while the video played, I saw averages of: Safari used 20, Firefox used 45, Chrome used 45.
So on that task at least Safari is still king, but Firefox is on par with Chrome.