Updated for audio+screen from single getUserMedia request for FF Nightly.

// Firefox is supporting audio+screen from single
// getUserMedia request
// audio+video+screen will become audio+screen for Firefox
// because Firefox isn't supporting multi-streams feature

Below snippet is sharing single video stream containing both audio/video
tracks; and target browser is joining with only audio. Screen can be
viewed on both chrome and Firefox. If you'll share from chrome, then it
will be making multiple getUserMedia requests.

// initiator from Firefox
initiator.session = {
---- screen: true,
----
audio: true
};

// participant in chrome or
Firefox
participant.onNewSession = function(session) {
----
session.join({ audio: true });
};