> First, I was able to solve my problem with acceleration in Firefox
> using this about:config option
> 
> media.ffmpeg.vaapi.enabled = true
> 
> Once I enabled it, in about:support I saw h264 hardware decoding
> support (but not h265 and my card is capable).

Setting media.ffmpeg.vaapi.enabled = true,
I still see h264 as Unsupported under Media in about:support.
Am I missing something? vainfo says

Trying display: x11
vainfo: VA-API version: 1.22 (libva 2.22.0)
vainfo: Driver version: Intel i965 driver for Intel(R) Sandybridge Desktop - 2.4.1
vainfo: Supported profile and entrypoints
      VAProfileMPEG2Simple            :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointEncSlice
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointEncSlice
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointEncSlice
      VAProfileH264StereoHigh         :	VAEntrypointVLD
      VAProfileVC1Simple              :	VAEntrypointVLD
      VAProfileVC1Main                :	VAEntrypointVLD
      VAProfileVC1Advanced            :	VAEntrypointVLD
      VAProfileNone                   :	VAEntrypointVideoProc


Jan