It's a bug in Firefox, it can be found here:
https://bugzilla.mozilla.org/show_bug.cgi?id=934512
There's also a workaround:
context = new AudioContext();
navigator.getUserMedia({ audio: true }, function(stream) {
// the important thing is to save a reference to the MediaStreamAudioSourceNode
// thus, *window*.source or any other object reference will do
window.source = context.createMediaStreamSource(stream);
source.connect(context.destination);
}, alert);