Firefox : hardware video decoding blocklisted on AMD Radeon RX 6600

I’m trying to enable hardware video decoding on firefox since several days, with no avail. I’m using firefox 109.0.1 and i have a Radeon 6600, as the title says. I’ve already installed the vaapi mesa package, and i’m using the AMDGPU kernel module driver.

The vainfo output is correct :

Trying display: wayland
Trying display: x11
vainfo: VA-API version: 1.17 (libva 2.17.1)
vainfo: Driver version: Mesa Gallium driver 22.3.5 for AMD Radeon RX 6600 (navi23, LLVM 15.0.7, DRM 3.49, 6.1.11-1-MANJARO)
vainfo: Supported profile and entrypoints
      VAProfileMPEG2Simple            :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileJPEGBaseline           :	VAEntrypointVLD
      VAProfileVP9Profile0            :	VAEntrypointVLD
      VAProfileVP9Profile2            :	VAEntrypointVLD
      VAProfileAV1Profile0            :	VAEntrypointVLD
      VAProfileNone                   :	VAEntrypointVideoProc

I’ve managed to enable hw video decoding on chromium, but only for vp9 profile 0.
I have followed several guides, changed several flags on about:config, but despite this firefox just “blocklist” the gpu on about:support :

Blocklisted by gfxInfo
Bloccato: codice non funzionante FEATURE_HARDWARE_VIDEO_DECODING_DISABLE

media.ffmpeg.vaapi.enabled

is true

media.ffvpx.enabled
media.rdd-vpx.enabled

are false

media.navigator.mediadatadecoder_vpx_enabled

is true as well.

Also

gfx.webrender.all
media.hardware-video-decoding.force-enabled

are true.