Earlier quoted context omitted.
You cannot achieve true E2E encryption on Firefox as it doesn't support insertable streams, so I guess this is where the limitation is. EDIT: bug on bugzilla https://bugzilla.mozilla.org/show_bug.cgi?id=1631263
Then how come Zoom works in FF?
More details: https://webrtchacks.com/zoom-avoids-using-webrtc/