It's really weird, because some people I talk to has never noticed a problem, yet I've been able to reproduce it (unwillingly) every time I've tried to install firefox on any macbook - different machines, different OS version, clean installs, you name it. It's just a matter of installing FF and using it lightly for some minutes (different websites too, or just showing the start page in the background) and the fans are already at full speed.

It makes me really confused because my current understanding on the bug doesn't seem to be compatible with the idea of only affecting some users - and to be clear I'm not casting doubt on your experience either, it's just hard to imagine how a performance issue regarding the compositor and screen redrawing could be affecting users differently.

Make a repro checklist and file a bug, if it's so easy to reproduce for you! Write down what you do, as you do it, and maybe devs could then repro it too!