Politics. There is a compile-time flag in firefox that allows it to use system codecs (for images as well as video). I ran a self-built Firefox for some time that had this enabled. Mainly to view media files on the local network, and because there was a time where people were putting up some files that chrome could open, but not firefox (I forgot what that format was - maybe it was even webp).

If you enable the flag (or, disable the built-in whitelist), then Firefox uses GStreamer (and I believe DirectShow on Windows or Quartz on macOS, but I'm not sure) to display the videos.

There also used to be the option of using MPlayer as a plugin, but NPAPI support was unfortunately removed.