SOLVED: "Playback failed due to a fatal player error." (Only in Firefox)

[TheDreadPirate]

Replace this

Code:

devices:
      - /dev/dri:/dev/dri

with this

Code:

devices:
      - /dev/dri/renderD129:/dev/dri/renderD129

Right now you are passing in both the Arc GPU and the iGPU in your CPU. Jellyfin does not currently have a way (yet) to tell QSV which GPU to use so it uses the first available GPU, renderD128. Which is going to be your iGPU.