The Bugzilla entry is five years old. Chrome can do this. It's annoying that Firefox can not.
Not being able to play .mkv files makes it impractical for some people to use Firefox. They need this feature to play videos from Plex or Jellyfin and to monitor their security cameras. These are just two examples of very common use cases. And in such cases users are forced to use a Chromium-based browser. They simply have no other choice.
That's why I think Firefox should add this feature as soon as possible.