Can't nail it down to firefox but it ... seems somehow Decoder/Picture-Related?
Last two lines:
Sep 06 20:45:02 xynphonie kernel: [drm] amdgpu_dm_irq_schedule_work FAILED src 5
Sep 06 20:45:04 xynphonie kernel: [drm] amdgpu_dm_irq_schedule_work FAILED src 5
/edit: Some websearch suggested "bad GPU" . I had a problem before and was thinking about a bad GPU (last year) and send the card to Sapphire. They send it back and were unable to find a problem. I can stress the card for days, w/o a problem. So i'm not sure if its rly a bad GPU if some peeps on web are suggesting it.