It is an MKV (Matroska) file, a container that can hold video, multiple audio tracks, and subtitles in one file.
If you are having trouble with the file, these resources will help: Firefox_40198241_PL.mkv
Official Mozilla Support for video playback issues. It is an MKV (Matroska) file, a container
To help me find a more specific "paper" for you, could you tell me: It is an MKV (Matroska) file
Use VLC Media Player or Elmedia Player , which support the MKV format natively.