Hardware video decoding does not work in linux period. It doesn't matter whether you play an mp4 or vp9 video.
It's true that Chrome/Chromium don't support it. [0]
Linux itself certainly supports it [1], and at least Mozilla have some progress towards it happening [2][3][4], if slow progress.
However, we used to have hardware video decoding via gstreamer and VLC within Firefox as plugins, using VAAPI and NPAPI, so it's still possible to get working today.
[0] https://bugs.chromium.org/p/chromium/issues/detail?id=463440
[1] https://wiki.archlinux.org/index.php/Hardware_video_accelera...
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=563206