Firefox - MP4 Cannot be Decoded

One constant issue I have with Firefox is the instability of playing videos outside of Youtube. Often when watching anime online, I would often encounter issues where skipping even a second would cause the video to stop playing. Hence why I have mention in my FAQ that I use Brave, a privacy-focused chromium browser, to stream anime.

While making my gallery featuring retro tech on neocities, I encountered a similar issue. Firefox was unable to loop a MP4 video I uploaded. Though Brave unsurprisingly could.

The simple solution to the problem was to convert the MP4 video into webm, an open and royalty-free media format which is the image you see on the top meanwhile the video on the bottom was encoded in HEVC (H.265) via ffmpeg (not ffmpeg-free). I was under the impression that Firefox supported HEVC but it turns out there are only limited support. Based on the Error displayed on the video and Firefox complaining decoding error on the Web conole, I knew Firefox was missing some type of decoder to play my MP4 file.

Placing about:support#media on the Firefox address bar reveals everything I needed to know:

Despite having HEVC en/de-coders on my Linux system, I needed to enabled them via Firefox configuration (about:config):

• media.hevc.enabled
• dom.media.webcodecs.h265.enabled

However, I opted to convert my MP4 file into webm to open access to all visitors.

HEVC is a patented code which based on Wikipedia which only waives royalties on software-only implementations (and cannot be bundled with hardware).

Note: “Prise en charge” is “Supported” and “Décodage logiciel” is “Software Decoder” in English

Note: Not a support on Neocities so the video is hosted on codeberg