(e.g., watch it, convert it, or find out who sent it?) MKV Files Explained - Adobe
It is an MKV (Matroska) file, a container that can hold video, multiple audio tracks, and subtitles in one file. Firefox_40198241_PL.mkv
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. a container that can hold video
If you are looking for "papers" regarding why Firefox generates or interacts with these files, check these sources: multiple audio tracks
To help me find a more specific "paper" for you, could you tell me: