If you haven't seen the release notes WebVTT has finally been released in Firefox 31. I'm super excited about this as it's the culmination of a lot of my own and countless others' work over the last two years. Especially since it has been delayed for releases 29 and 30.

That being said, there are still a few known major bugs with WebVTT in Firefox 31:

• TextTrackCue enter, exit, and change events do not work yet. I'm working on getting them done now.
• WebVTT subtitles do not show on audio only elements yet. This will probably be what is tackled after the TextTrackCue events (EDIT: To clarify, I meant audio only video elements).
• There is no support for any in-band TextTrack WebVTT data yet. If your a video or audio codec developer that wants in-band WebVTT to work in Firefox, please help out :-).
• Oh, and there is no UI on the HTML5 Video element to control subtitles... not the most convenient, but it's currently being worked on as well.

I do expect the bugs to start rolling in as well and I'm actually kind of looking forward to that as it will help improve WebVTT in Firefox.

http://rickeyre.ca/2014/07/23/webvtt-released.html