Sounds absolutely terrible on Firefox 18.0.2/Linux... I think it's sad MIDI isn't in the Web Audio API, the code for 95% of this is written already, just not accessible. The web really needs to reinvent everything in JavaScript I guess.
It's sad that everything HAS to be re-invented, but I am frequently excited to see what can be done with ES5. I'm eager to see what ES6 brings us.