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Firefox supports H.264 on Mac via the QuickTime plugin.

Firefox supports H.264 on Linux via the Flash plugin

Flash has historically always been used to deliver H264 to Firefox, though. That's not quite the same as the browser supporting it in <video>.

Firefox doesn't support H264, but it can offload the decoding to the OS. So if your OS doesn't have it, it won't work. This affects Windows XP, which still has substantial users.

The writing's on the wall, though, so you can just use something like MediaElement.js' Flash fallback until Mozilla ships OS H.264 support for other platforms. That's a much better situation than having to double your media transcoding & storage costs.