Newer firefox made webrender mandatory, and software webrender in VMs performs really bad for me.
debian-stable was stuck on firefox-esr-78 for a while which had the old, fast rendering, that might explain why debian firefox was a better experience than fedora firefox.
Workarounds: Make an online movie [in browser] to play correctly - #2 by airelemental
Related bug: 1738247 - Videos stutter with Webrender Software on a Xen VM with no hardware acceleration compared to Firefox 78 esr