Hello,
I’m assuming you mean that the file Does play in Chrome, but Not in Firefox?
Firefox does not natively support the H.264 codec, which means that it cannot play normal video files unless you use a plugin like flash player. Very recently, however, Cisco announced that they have licensed the H.264 codec and are providing it for free. Firefox has agreed that it will (finally) support H.264 natively using this Cisco license. I do not know, however, when Firefox will actually roll out that support.
Bryan