MP4 videos fail to play with the following error messages: # No video with supported format and MIME type found. # Video can't be played because the file is corrupt. # NS_ERROR_DOM_MEDIA_FATAL_ERR (0x806e0005) # Error no decoder found for audio/mp4a-latm example video #1 on [https://tekeye.uk/html/html5-video-test-page https://tekeye.uk/html/html5-video-test-page] other notes: * issue began a 1-2 weeks ago * issue does not exist on chrome browser * MP4 videos can be downloaded and played successfully in applications other than firefox * other video formats play successfully in firefox * issue persists in troubleshoot mode * issue persists with hardware acceleration on/off * issue persists with tracking protection on/off * media.mp4.enabled is set to true