more accurately, OS media decoders are now supported on firefox. Firefox isn't actually providing support for h.264, they're just exposing your operating system's video decoders. so if your OS doesn't have h.264 support, you're not going to have it in firefox.
As far as I know, the only reasonably common desktop OS to lack H.264 support out of the box is Windows XP, and Mozilla plans to use Flash to decode H.264 video there.
Well, I guess Linux also doesn't support H.264 unless you're in a jurisdiction that doesn't allow software patents, or you're willing to be a scofflaw, but there's not much Mozilla can do about that. :(