Official Mozilla Support for video playback issues.
It is an MKV (Matroska) file, a container that can hold video, multiple audio tracks, and subtitles in one file. Firefox_40198241_PL.mkv
If the file won't open, it might be an incomplete download. You can try a tool like MKVToolNix to inspect or repair the file headers. 📄 Documentation for Context Official Mozilla Support for video playback issues
If you are having trouble with the file, these resources will help: a container that can hold video
Use VLC Media Player or Elmedia Player , which support the MKV format natively.
(e.g., your Downloads folder, a specific website, or a program directory?)