As part of the ongoing WebRTC spec compliance work, Firefox 63 and later will use a string ID (MID) instead of an integer level (m-line index) as the identifier for media transports. Because of this, SDP offers should also be spec compliant, requiring the
mid property. The simplest solution here is adding
a=mid:0 to offers, according to a Mozilla engineer’s bug comment.
Google Meet and Hangouts were broken after the change was made, but Google has already fixed the issue. TextNow is also known to be broken.