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 .
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)