Repro showing that relaying messages via a worker does not work in FireFox

channels.html

<script>
  console.log('Starting up');

  var receiverCode = '(' + (() => {
      console.log('Hello from receiver');

      self.onmessage = (msg) => {
          if (typeof msg.data === 'string') {
              console.log('received from main:', msg.data);
          } else {
              var relay = msg.data.relay;
              console.log('receiver received port');

              relay.onmessage = (m) => {
                  console.log('received from relay:', m.data);
              };
          }
      };
  }).toString() + ')()';

  var relayCode = '(' + (() => {
      console.log('Hello from relay');

      self.onmessage = (msg) => {
          var sender = msg.data.sender;
          var receiver = msg.data.receiver;
          console.log('relay received ports');

          sender.onmessage = (m) => {
              console.log('worker relaying', m.data);
              receiver.postMessage(m.data);
          };
      };
  }).toString() + ')()';


  var senderCode = '(' + (() => {
      console.log('Hello from sender');

      self.onmessage = (msg) => {
          var relay = msg.data.relay;
          console.log('sender received port');

          relay.postMessage('hi via relay');
          self.postMessage('hi via main');

          // Spin to emulate synchronously waiting for the message to be
          // received.
          while (1) {}
      };
  }).toString() + ')()';

  var uri = (code) => 'data:text/javascript,' + encodeURIComponent(code);

  var receiver = new Worker(uri(receiverCode), {name: 'receiver'});
  var relay = new Worker(uri(relayCode), {name: 'relay'});
  var sender = new Worker(uri(senderCode), {name: 'sender'});

  sender.onmessage = (m) => {
      console.log('main relaying', m.data);
      receiver.postMessage(m.data);
  };

  console.log('sending ports');

  var senderChannel = new MessageChannel();
  var receiverChannel = new MessageChannel();

  sender.postMessage({
      relay: senderChannel.port1
  }, [senderChannel.port1]);

  relay.postMessage({
      sender: senderChannel.port2,
      receiver: receiverChannel.port1
  }, [senderChannel.port2, receiverChannel.port1]);

  receiver.postMessage({
      relay: receiverChannel.port2
  }, [receiverChannel.port2]);

</script>