Something weird is happening with my website's MP4 videos in Firefox (39.0.3), using the videojs html5 player. For example, in a 4-minute video: sometimes there is no audio; sometimes the audio starts but cuts out after 30 seconds; sometimes the last 30 seconds of audio starts playing after the first 30 seconds. This behaviour is only seen (or heard) in files accessed off the server - accessed off the C-drive, they play fine. The MP4-with-audio-problems was created from a .MOV file using Handbrake 0.10. I noticed that other MP4s I did last year (with Handbrake 0.09) do not have audio problems in Firefox. So it may be a Handbrake issue? Anyway, workaround! I converted the same .MOV to .M4V (using Handbrake) and the audio problems have disappeared. It's odd, because M4V is supposedly the exact same file as the MP4 - only the extension is different. But it works.