Running any of those on Firefox, vs. other browsers, is a lot hotter.
I did a post about it and according to Apple's "Energy Usage" Firefox was like 50x higher than Safari and 30x higher than Chrome on the same sort of streaming call.
Try it for yourself next time you're on a Zoom call.
Firefox is my browser of choice, but so annoying that they don't take battery use / optimization seriously.
I want my Mac Hardware, it feels nice. I want to use Firefox, I trust it and I like that I can automatically clear cookies and history on close... The fact that Chrome pushes you to sign in is so fundamentally offensive to me. Just super-creepy. So I want Firefox on Mac, but I have no strong feelings towards Gecko one way or the other, if that's what's causing my computer to over heat... wish they'd just swap it out for Webkit.
Firefox/Gecko has an excellent rendering engine but Mozilla has often been late to the party with hardware acceleration on Mac.
It's also worth noting that Safari doesn't necessarily support all the same video codecs as Firefox, so for example Zoom might be using hardware accelerated h264 in Safari whereas in Firefox might use a more advanced codec (e.g. VP9) for which your MacBook might have no hardware acceleration capability. If this is the case, Firefox ought to give users the option to disable support for these non-accelerated codecs when running on battery power.
It's not a Webkit issue. The parent comment is confusing iOS with macOS and the rendering engine used by Firefox on macOS.
You're also mixing up symptoms. If all browsers were forced to use the same rendering engine, you'd see little divergence of CPU/energy usage. The user is seeing a wild divergence—which is what you might expect when browsers are using different rendering engines.
You're also mixing up talking points. People have accused Safari of being many things, but being profligate with CPU/energy usage is certainly not one of them.