Adding captions and subtitles to HTML5 video - Developer guides
firefox firefox's implementation was completely broken due to a bug, leading to mozilla turning off webvtt support by default (you can turn it on via the media.webvtt.enabled flag.) however, this bug looks to have been fixed and webvtt support re-enabled as of gecko 31, so this will not be a problem for firefox final release users for much longer (on gecko 29 as of the time of this writing) this has been fixed as of firefox 31, and everything works as it should.