The 08 version of the jsep draft requires m-lines (in the offer) to be added as follows: First sort by stream-id, based on the order in which the streams were added. Within a given stream, sort based on alphabetical order of media type. Within a given stream/type, sort based on order in which the tracks were added. This means (among other things) that if a video-only stream is added, then an audio-only stream is added, we will have video before audio, which pre-sdparta Firefox cannot handle. We probably want to fix this once 38 is ESR.

ESR 38 is out since quite some time now :-)

Perhaps this issue can be revisited by Mozilla staff. For in Firefox Nightly 62.0a1 I'm experiencing an issue where I have my WebRTC page settings selected as "Use Video(checked", "No Microphone", and "No Camera", which should be acceptable, but FF console errors says: DOMException: Answer and offer have difference media types at m-line 0 And looking in my FreeSWITCH logs (and in Firefox console), Firefox is sending a SDP with m=video, followed by m=audio... I believe conventional practice is to have audio first, follow by video. It seems when "No Microphone" is selected, Firefox is generating an initial offer with m=video/a=sendrecv first, even though m=audio/a=recvonly is also present... A complete workup of FreeSWITCH logs contain SDP dialog is here. https://docs.google.com/spreadsheets/d/1Ep5CfbP9hYD1f5Dlkf6hozv5DdVDXZ5tpiK-OzOFLbA/edit?usp=sharing

So the spec has changed here. As of the most recent transceivers-based spec, the m-sections appear in the order that their transceivers were added. If both a transceiver and a datachannel are added before a subsequent SDP is generated, the application m-section appears after the audio/video one.