[randomdata]

> Do codecs need to be supported by the browser itself?

Not necessarily. The browser could defer to licensing established by the operating system vendor, but Firefox places the expectation upon itself to have parity across platforms and to not support encumbered technologies.

> Linux does have h.265 support at least in mpv.

And if you've negotiated the licensing fees you can even use it, but chances are... Microsoft and Apple have dealt with the licensing for you on their platforms, so the ballgame is different there.