Something is still extremely funky with video on macOS Firefox. Playing MP4s on YouTube still uses 200-400% more power than on Safari, and that's with transparent window disabled. Hell, Twitch turns my 2015 MBP into a vacuum cleaner but Safari barely breaks a sweat.
Have you tried using a Nightly build of Firefox? There's recently been some work to use CoreAnimation on MacOS to reduce power consumption that hasn't made it to the Beta or Release channels yet. (See: https://bugzilla.mozilla.org/show_bug.cgi?id=1429522)
There's also [0] which investigates streamed video including YouTube specifically.
While for all major platforms even including Windows on ARM64 the performance is good for VP9, and somewhat degraded for high resolution H264 videos, the results for Mac are bad across all of them. It's not even possible to playback videos with a 480p resolution, without being affected by a lot of framedrops.
In those 15s of playback a 480p VP9 video produces around 5 dropped frames, a 720p and 1080p video already ~45 dropped frames. Really worse it is with 4k@60fps videos which have more than 400 dropped frames even with a playback speed of 1.0x!