You can issue a bug report on https://bugzilla.mozilla.org/home Sometimes bugs are fixed really fast, sometimes never. Just give it a try.

There could be something related to FAN thresholds that resides in BIOS directly thus starting using GPU just slightly over some minimal limit might raise FAN speeds significantly and not be related to temperature raise as there is some inertia between temperature readings and real temperature inside GPU core.

I'm also experiencing FAN noise raise on my Ryzen APU based laptop.

i mostly stopped bothering with in-browser video when mpv wiht youtube-dl integration came around. either drag the url to the player window or use an open-with addon.

hardware decoding, no ads, buffering as i please