Earlier quoted context omitted.
In Firefox the envelope release is broken, and overall performance seems worse compared to Chrome, for reasons I haven't really bothered to investigate deeply. It mostly works in Safari, but Safari does something different with garbage collection of audio nodes, so the note-stealing behavior doesn't work right. Basically, use Chrome for now if you want it to play nice. WebMIDI is only supported in Chrome anyhow. Also…
Yeah we're fixing this as we speak (I work on Web Audio API at Mozilla), it should work fine in Nightly in a couple weeks, worst case.
Ha, thank you Paul! Glad to know it's probably not my fault ;)