2025-04-08, 10:40 PM
I was going to make a post about this myself, and was happy to see someone else chime in on this topic. I can confirm via about:support that hardware decoding for HEVC is indeed enabled on Firefox 137.0 on Linux via VA-API. Pretty exciting news. 
At the time of writing, Firefox supports HEVC hardware decoding on Windows (134.0+), macOS (136.0+), and now Linux (137.0+). No idea about other platforms like FreeBSD and the like, but it seems support is finally available across all major desktop OSes.
It would be great if the H.265 entry in the "Firefox" column in the Codec Support table could be updated in a similar manner to the "Chrome" entry, to indicate that Firefox 137.0+ will support HEVC decoding when HEVC hardware decoding is available.
At the time of writing, Firefox supports HEVC hardware decoding on Windows (134.0+), macOS (136.0+), and now Linux (137.0+). No idea about other platforms like FreeBSD and the like, but it seems support is finally available across all major desktop OSes.
It would be great if the H.265 entry in the "Firefox" column in the Codec Support table could be updated in a similar manner to the "Chrome" entry, to indicate that Firefox 137.0+ will support HEVC decoding when HEVC hardware decoding is available.