My starting point.
Mockup showing track.onenabled proposal in WebRTC call in Firefox Developer Edition
How to connect a WebRTC call in Nightly before granting camera access.
Shows modern WebRTC use with parallel setup in Firefox Nightly.
For it to work, you must add "jsfiddle.net" to media.getusermedia.screensharing.allowed_domains. You might also want to set media.navigator.permission.disabled as well. Be careful!
Try to adapt WebRTC demo to work on both Canary and Nightly