Firefox with uBO [0] and Auto Tab Discard [1] is pretty good on the battery. I often go 10h without a charge on my M1 Air and usually there's still a bit to spare.
I have, but nothing stood out. I even tried Firefox Profiler (which is pretty good btw, kudos to the team), and it looks like DOM processing and renderer both spend a lot of CPU. But alas I'm not a frontend developer and have no incentive to dive deeper :)