My guess is that the OP doesn't have hardware acceleration for Firefox on his GPU configuration. You probably don't either with your numbers. I have about 3-4% CPU usage on GitHub.com using Firefox, and my GPU is at about 20% load from the 3D being drawn.

after reading your comment I checked again and realized that activity monitor has a column for %GPU as well under the CPU tab. So here's the eyeball average after watching the GitHub page for about a minute.

Firefox 88 (no extensions, private mode):

CPU% 55 (avg) 61 (peak)

GPU% 65 (avg) 77 (peak)

Safari (no extensions):

CPU% 33 (avg) 40 (peak)

GPU% 33 (avg) 40 (peak) <- same numbers for both CPU and GPU in safari.

Maybe it's running on a low power core?

Another M1 datapoint,

Chrome: ~20% Safari: ~35% Firefox: ~45%