From my understanding of the WebRTC spec, as long as your TURN/STUN server is trusted (which it is, the CA and relay are both run by Mozilla) it doesn't matter if the network is untrustworthy.
WebRTC by itsself is not MITM resistant
WebRTC by itsself is not MITM resistant
In other words, can Telefonica make it easy to spy on all Firefox Hello talks, or only on the browser-to-phone ones? (which is to be expected, I guess).