I believe one has already been ported using Emscripten - but this was a while ago, I'd be curious to see how fast it runs now with Asm.js: https://github.com/mbebenita/Broadway

The reverse has been done: WebM has been ported to JavaScript with Emscripten, which means all browsers (IE9+) can play WebM to some degree.

http://badassjs.com/post/17218459521/webm-and-webp-hand-port...

Fairly easily if you start with Broadway: it’s an Emscripten port of Android’s H264 software decoder that writes to a WebGL canvas.

https://github.com/mbebenita/Broadway

Wasn't Firefox 20 supposed to have H264 support?

Yes. But it hasn't somehow on Mac, because I have been reading it's already there on Windows(never tried myself though).