--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -149,17 +149,16 @@ browser.Context = class {
     // been fully processed. The commands get flushed after a new browser
     // has been registered.
     this.pendingCommands = [];
     this._needsFlushPendingCommands = false;
 
     // We should have one frame.Manager per browser.Context so that we
     // can handle modals in each <xul:browser>.
     this.frameManager = new frame.Manager(driver);
-    this.frameRegsPending = 0;
 
     // register all message listeners
     this.frameManager.addMessageManagerListeners(driver.mm);
     this.getIdForBrowser = driver.getIdForBrowser.bind(driver);
     this.updateIdForBrowser = driver.updateIdForBrowser.bind(driver);
   }
 
   /**

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -133,16 +133,17 @@ this.GeckoDriver = function(appId, serve
   // points to current browser
   this.curBrowser = null;
   // topmost chrome frame
   this.mainFrame = null;
   // chrome iframe that currently has focus
   this.curFrame = null;
   this.mozBrowserClose = null;
   this.currentFrameElement = null;
+  this.frameRegsPending = 0;
   // frame ID of the current remote frame, used for mozbrowserclose events
   this.oopFrameId = null;
   this.observing = null;
   this._browserIds = new WeakMap();
 
   // The curent context decides if commands should affect chrome- or
   // content space.
   this.context = Context.Content;
@@ -499,20 +500,20 @@ GeckoDriver.prototype.whenBrowserStarted
       // return to the server. If the messageManager is a message broadcaster
       // with no children, we don't have a hope of coming back from this
       // call, so send the ack here. Otherwise, make a note of how many
       // child scripts will be loaded so we known when it's safe to return.
       // Child managers may not have child scripts yet (e.g. socialapi),
       // only count child managers that have children, but only count the top
       // level children as they are the ones that we expect a response from.
       if (mm.childCount !== 0) {
-        this.curBrowser.frameRegsPending = 0;
+        this.frameRegsPending = 0;
         for (let i = 0; i < mm.childCount; i++) {
           if (mm.getChildAt(i).childCount !== 0) {
-            this.curBrowser.frameRegsPending += 1;
+            this.frameRegsPending += 1;
           }
         }
       }
     }
 
     if (!Preferences.get(CONTENT_LISTENER_PREF) || !isNewSession) {
       // load listener into the remote frame
       // and any applicable new frames
@@ -605,21 +606,21 @@ GeckoDriver.prototype.registerPromise = 
   const li = "Marionette:register";
 
   return new Promise(resolve => {
     let cb = msg => {
       let wid = msg.json.value;
       let be = msg.target;
       let rv = this.registerBrowser(wid, be);
 
-      if (this.curBrowser.frameRegsPending > 0) {
-        this.curBrowser.frameRegsPending--;
+      if (this.frameRegsPending > 0) {
+        this.frameRegsPending--;
       }
 
-      if (this.curBrowser.frameRegsPending === 0) {
+      if (this.frameRegsPending === 0) {
         this.mm.removeMessageListener(li, cb);
         resolve();
       }
 
       // this is a sync message and listeners expect the ID back
       return rv;
     };
     this.mm.addMessageListener(li, cb);