As an example: http://m.joystiq.com/2014/02/16/plants-vs-zombies-garden-warfare-live-action-trailer-brings-the/?ncid=txtlnkusaolp00000598 There is an MP4 video on the site, which plays fine in Chrome and Android browser. In Firefox, I get to see the preview image with the grey "play" symbol over it, but once I hit it all I get is black screen with audio. Most of the messages I've seen around this type of error talk about Flash plug-in installs, etc. AFAIK, mp4 isn't flash. I've disabled all my add-ons. I'm on an HTC One, Android 4.4.2. Any suggestions?