It's because the video is using HEVC/H.265 which isn't enabled in Firefox. From some rudimentary research, it may be possible to modify some settings in about:c

It's because the video is using HEVC/H.265 which isn't enabled in Firefox. From some rudimentary research, it may be possible to modify some settings in about:config to enable it. > Supported for devices with hardware support (the range is the same as Edge) on Windows only. Enabled by default in Nightly and can be enabled via the media.wmf.hevc.enabled pref in about:config. 10-bit or higher colors are not supported. https://caniuse.com/hevc https://caniuse.com/hevc