When receiving a new incoming stream from server, the function OnIncomingBidirectionalStreamAvailable or OnIncomingUnidirectionalStreamAvailable should be called.

// Events related to a WebTransport session.
interface WebTransportSessionEventListener {
  // This is used to let the consumer of nsIWebTransport know that the
  // underlying WebTransportSession object is ready to use.
  void OnSessionReady(in uint64_t aSessionId);
  // This is used internally to pass the reference of WebTransportSession
  // object to WebTransportSessionProxy.
  void OnSessionReadyInternal(in WebTransportSession aSession);
  void OnSessionClosed(in uint32_t aErrorCode,
                       in ACString aReason);

  // When a new stream has been received.
  void OnIncomingBidirectionalStreamAvailable(in nsIWebTransportBidirectionalStream aStream);
  void OnIncomingUnidirectionalStreamAvailable(in nsIWebTransportReceiveStream aStream);

  // When a new datagram has been received.
  void OnDatagramReceived(in AUTF8String aDatagram);

  void OnStatsAvailable(in WebTransportStats aStats);
};

https://hg.mozilla.org/mozilla-central/rev/1b39337257b7