--- a/devtools/server/main.js
+++ b/devtools/server/main.js
@@ -1058,17 +1058,17 @@ var DebuggerServer = {
 
       connection.setForwarding(prefix, childTransport);
 
       dumpn("establishing forwarding for app with prefix " + prefix);
 
       actor = msg.json.actor;
 
       let { NetworkMonitorManager } = require("devtools/shared/webconsole/network-monitor");
-      netMonitor = new NetworkMonitorManager(frame, actor.actor);
+      netMonitor = new NetworkMonitorManager(mm, actor.actor);
 
       deferred.resolve(actor);
     }).bind(this);
 
     // Listen for browser frame swap
     let onBrowserSwap = ({ detail: newFrame }) => {
       // Remove listeners from old frame and mm
       untrackMessageManager();

--- a/devtools/shared/webconsole/network-monitor.js
+++ b/devtools/shared/webconsole/network-monitor.js
@@ -1559,26 +1559,23 @@ NetworkEventActorProxy.prototype = {
 })();
 
 /**
  * The NetworkMonitor manager used by the Webapps actor in the main process.
  * This object uses the message manager to listen for requests from the child
  * process to start/stop the network monitor.
  *
  * @constructor
- * @param nsIDOMElement frame
- *        The browser frame to work with (mozbrowser).
+ * @param nsIMessageManager mm
+ *        The message manager for the browser we're filtering on.
  * @param string id
  *        Instance identifier to use for messages.
  */
-function NetworkMonitorManager(frame, id) {
+function NetworkMonitorManager(mm, id) {
   this.id = id;
-  // Get messageManager from XUL browser (which might be a specialized tunnel for RDM)
-  // or else fallback to asking the frameLoader itself.
-  let mm = frame.messageManager || frame.frameLoader.messageManager;
   this.messageManager = mm;
   this.onNetMonitorMessage = this.onNetMonitorMessage.bind(this);
   this.onNetworkEvent = this.onNetworkEvent.bind(this);
 
   mm.addMessageListener("debug:netmonitor:" + id, this.onNetMonitorMessage);
 }
 exports.NetworkMonitorManager = NetworkMonitorManager;