I suggest reporting a bug to Bugzilla. The positive side effect will be you will see how efficiently they handle such reports. And you will get it fixed.

I'm pretty sure they are aware of it.

I have the same issues on Linux. It's really fast, but often it hogs one core completely and the CPU does a lot of work, sending the fans in overdrive. Especially when videos are involved.

Let's hope things get ironed until a stable release in november

As said in sibling: Open a bug report at https://bugzilla.mozilla.org/

Mention Quantum Flow and "performance" in it. Make sure to attach a performance profile with https://perf-html.io/ using the linked add-on there.

I had many cases where the mozilla devs were glad about such bug reporst (especially if you are on Linux - I am too :) - which is not so often used and reported like Windows or Mac)

Done: https://bugzilla.mozilla.org/show_bug.cgi?id=1407536

We're aware that there seems to be a problem, but we can't reproduce it yet: https://bugzilla.mozilla.org/show_bug.cgi?id=1404042 Any additional data points, test cases etc. are appreciated.

I filed a new report here, with perf.html attached: https://bugzilla.mozilla.org/show_bug.cgi?id=1407536