Was reading an answer to the question about fresh Amazon tablet and saw this. They used up the question as a hook to promote their browser. Pretty sure I'm avoiding Brave from now on.
Q: Does the fire max 11 have YouTube app on it. A: I have installed Google Playstore , but I skip installing Youtube app . There is better option . Brave Browser in Playstore !! That thing has AWESOME popup Blocker and it's own integrated Youtube Ads and other Nuisances blocker . My point ? You don't need youtube on Fire max , You don't need Google Maps , You don't need Facebook App . Just Use Brave .
Firefox runs really poorly for twitch and youtube videos for me in linux; mainly in CPU. Brave will use maybe 10-20% of my cpu for a single twitch stream whereas firefox uses about 80%. I use firefox for all my browsing, but for videos it just doesn't seem to work well... Perhaps a different chromium browser would be better as you say.
Sounds like your Firefox is not using hardware acceleration for displaying videos. It does support it, but whether it will be enabled on Linux depends on a bunch of factors (distro/packages installed, cpu/gpu vendor/drivers, some weird settings etc etc)