Why doesn't MediaStream.removeTrack() work in Firefox?

Question

JavaScript

Does anyone know why a number of the MediaStream function in firefox does not work? Like cloning a stream or deleting from a stream? Moreover, they are documented https://developer.mozilla.org/en-US/docs/Web/API/M...
However, open the example in Firefox - jsfiddle.net/agaRe/840 In it, I remove the video stream from the general stream , there will be an error in firefox, it works fine in chrome\opera, we will hear only sound.