If you are running Firefox 45 on Windows with multiple network interfaces (if you have vmware or virtualbox installed it will create multiple virtual nics). There is the potential for Firefox's WebRTC stack to select the wrong interface for STUN messages leaving you with no media & an 'ICE candidates fail' error message.
For more information: https://bugzilla.mozilla.org/show_bug.cgi?id=1107702
1. Upgrade to Firefox 46 (FF will do this automatically if you click on about within firefox).
2. Disable and re-enable the interface.