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Different commenter here, but same issue as the parent. I'm on a 2016 MBP. When I use Chrome, my fans stay off or at around 1200rpm at all times unless I open a video or a Twitch stream, at which point they jump up to around 1400rpm and the highest the CPU temp gets is around 50c. On Firefox, my fans will randomly jump up to 3000rpm when doing only normal web browsing, even on simple sites like HN. I'm using FF right…
Exactly the same with me: I tried switching to Firefox 63 last week on a 2015 MBP running MacOS 10.11, and compared to Chrome it seems to use a lot more CPU and thus makes my machine run noticeably hotter. I guess this might be due to the new Rust layout engine I've heard about which is more parallelised?, but even just having a single tab playing youtube or a gif uses more CPU in Firefox, so maybe it's something wit…
My investigations (their bugzilla, the news about their company, I've been following for a long time) point to the opposite: it's the "old code" which nobody wants to improve: it doesn't help if the "new" code is fast to draw Some detail when the "old" decides to draw or update the stuff that doesn't even has to be updated or drawn. And it's not just drawing too, it's all the processing that happens during the life of the page with the "moving" images. Nobody, as "especially managers in Firefox." There was a number of developers in some Asian country, paid by Firefox, who were in charge for fixing in the old code, and they were simply fired. It seems it's not "strategic" for the managers to "fix" things, only to "experiment" with "new" things like ads or "new technologies" or whatever.
The "more CPU and GPU use" of Firefox is absolutely observable on any platform, not only on Macs. There are also the bugs submitted, but it doesn't seem that there's any priority in fixing them. From their perspective "it works, it's just that those who measure such use notice it." It's of course not so. The battery is empty earlier, the notebooks are hotter, the response lags. There are reported bugs demonstrating exactly what you observed: compared to any other browser even a single video on the "modern" site in Firefox needs much more processing power. It’s easy to reproduce in Windows too.
But obviously fixing the performance problems is not priority for the managers, when there's anything "new and shiny." Because they don’t see that as serious bugs: “the same page uses 10% of the max possible power in Chrome but 20% in Firefox.” “Who cares?” It’s too long-term goal.