How to play MKV videos in Mozilla FIrefox?
Unfortunately its simply not supported.
There is bug on Mozilla's Bugzilla - https://bugzilla.mozilla.org/show_bug.cgi?id=1422891 but without much of progress here.
.Mkv and/or other video files can be played through firefox and firefox mobile by either right-click/long-press on video link for play options then choose the video player you know sorts the file example VLC player on Android market.