It doesn't, this is also the reason that streamers like Nvidia Shield or Apple TV are the only two choices if you want to view 4K content at all.

I have been using firefox on linux for a little more than a decade now and haven't realize I was missing on anything so I guess it's probably not a real problem.

Netflix et al work on Linux but are limited to 480p.

Without the propietary Widevine, maybe.

720p

Which is not a big deal when you are watching on a laptop screen.or via a projector.

and yet ~some devices are constantly pwned, and pristine UHD WEB-DL's are being ripped automatically.

> Does Widevine CDM work on Firefox on Linux?

It does, but it's L3-only (software-based), not L1 (hardware-based) [0]. Streaming providers can then decide which content they'll let you access depending on the level. Speaking from experience, some providers work perfectly (full HD content with no issues), others only give you a really low-resolution stream, and others refuse to work entirely.

> If so, why would Google allow this but not for other OSS browsers?

When EME [1] was first released, Firefox had ~10% market share, and it would look pretty bad for Google to exclude another major browser maker. Smaller browsers don't have the political clout necessary to convince Google to give them access.

[0]: https://en.wikipedia.org/wiki/Widevine#Architecture

[1]: https://en.wikipedia.org/wiki/Encrypted_Media_Extensions