Just did some tests on Firefox 83 and DOM reflow time grows exponentially with the depth of DOM tree. With grid elements of depth 10 single reflow takes seconds on my Ryzen laptop while on chromium is not noticable at all.
Can you post a link to a test case?
Did already profiling and posted a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1682686
It's very very very simple and quick, there is a video demo and a text explaining what to do, and then you can open a bug at https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&comp..., with the profile link (you can login with a github account or email). This will be triaged and hopefully we'll understand what's up. Please use a Nightly build or a Mozilla-provided build when doing so (so there are symbols). It's just a matter of downloading and untaring the tarball.
Please also attach a copy of `about:support` raw data (button at the top), so we can understand what your configuration is: desktop Linux systems can be pretty diverse and sometimes it isn't clear what works, what doesn't and why.
If you're playing 480p, then youtube might be serving you AV1, and this is (for now) using the CPU (trading a lot less bandwidth for increased CPU usage), you can check the codec it's using by right clicking on the video and clicking on `Stats for nerds`, there is the codec string. You can adjust your codec preferences in your youtube profile I believe.