Use VLC Media Player or Elmedia Player , which support the MKV format natively.
If you are having trouble with the file, these resources will help: Firefox_40198241_PL.mkv
Technical details on the MKV Standard .
(e.g., your Downloads folder, a specific website, or a program directory?) Use VLC Media Player or Elmedia Player ,
While Firefox has recently added better support for MKV playback, you may still need to use a dedicated player if the video stream inside is encoded with a specific codec like HEVC. your Downloads folder
To help me find a more specific "paper" for you, could you tell me: