Not until a few years ago, Firefox on macOS just kept spinning the fans because it used that much CPU, for no reason. I stopped using FF since then, haven't bothered to install it again, might do it in the near future.
I tried using Firefox last year, I did for a few months. But it's the slowest out of all the major browsers on macOS. So I'm back to using Safari with Chrome/Edge for the few sites that don't work in it.