Comment 53 for bug 434476

meanwhile, workaround:
media.ogg.enabled;false
media.opus.enabled;false
media.raw.enabled;false
media.webm.enabled;false
media.windows-media-foundation.enabled;false
media.directshow.enabled;false
media.navigator.enabled;false
media.autoplay.enabled;false
media.windows-media-foundation.play-stand-alone;false
media.windows-media-foundation.use-dxva;false
media.peerconnection.enabled;false
media.wave.enabled;false

most players then fall back to Flash, and since FF currently is no media player because of this bug, its the best thing one can do ....