UPDATE: Looks like it not working is Mozilla's mistake, not your browser's. Using fiddler shows that the mime type Mozilla is using is binary/octet-stream, but

UPDATE: Looks like it not working is Mozilla's mistake, not your browser's. Using fiddler shows that the mime type Mozilla is using is binary/octet-stream, but it looks like the correct mime-type should be text/vtt. This is documented on Mozilla's page at https://developer.mozilla.org/en-US/docs/Web/API/Web_Video_Text_Tracks_Format https://developer.mozilla.org/en-US/docs/Web/API/Web_Video_T... Also look at http://dev.w3.org/html5/webvtt/#refsRFC3629 http://dev.w3.org/html5/webvtt/#refsRFC3629 and search "mime"