--- a/browser/components/extensions/test/browser/browser_ext_contentscript_connect.js
+++ b/browser/components/extensions/test/browser/browser_ext_contentscript_connect.js
@@ -15,31 +15,33 @@ add_task(function* () {
       let port_messages_received = 0;
 
       browser.runtime.onConnect.addListener((port) => {
         browser.test.assertTrue(!!port, "port1 received");
 
         ports_received++;
         browser.test.assertEq(1, ports_received, "1 port received");
 
-        port.onMessage.addListener((msg, sender) => {
+        port.onMessage.addListener((msg, msgPort) => {
           browser.test.assertEq("port message", msg, "listener1 port message received");
+          browser.test.assertEq(port, msgPort, "onMessage should receive port as second argument");
 
           port_messages_received++;
           browser.test.assertEq(1, port_messages_received, "1 port message received");
         });
       });
       browser.runtime.onConnect.addListener((port) => {
         browser.test.assertTrue(!!port, "port2 received");
 
         ports_received++;
         browser.test.assertEq(2, ports_received, "2 ports received");
 
-        port.onMessage.addListener((msg, sender) => {
+        port.onMessage.addListener((msg, msgPort) => {
           browser.test.assertEq("port message", msg, "listener2 port message received");
+          browser.test.assertEq(port, msgPort, "onMessage should receive port as second argument");
 
           port_messages_received++;
           browser.test.assertEq(2, port_messages_received, "2 port messages received");
 
           browser.test.notifyPass("contentscript_connect.pass");
         });
       });
 

--- a/toolkit/components/extensions/ExtensionUtils.jsm
+++ b/toolkit/components/extensions/ExtensionUtils.jsm
@@ -1257,17 +1257,18 @@ Port.prototype = {
       postMessage: json => {
         this.postMessage(json);
       },
       onDisconnect: new EventManager(this.context, "Port.onDisconnect", fire => {
         return this.registerOnDisconnect(() => fire.withoutClone(portObj));
       }).api(),
       onMessage: new EventManager(this.context, "Port.onMessage", fire => {
         return this.registerOnMessage(msg => {
-          fire(msg);
+          msg = Cu.cloneInto(msg, this.context.cloneScope);
+          fire.withoutClone(msg, portObj);
         });
       }).api(),
     };
 
     if (this.sender) {
       publicAPI.sender = this.sender;
     }