--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1627,20 +1627,22 @@ GeckoDriver.prototype.setWindowHandle = 
     this.startBrowser(winProperties.win, false /* isNewSession */);
 
     if (registerBrowsers && browserListening) {
       await registerBrowsers;
       await browserListening;
     }
 
   } else {
-    // Otherwise switch to the known chrome window, and activate the tab
-    // if it's a content browser.
+    // Otherwise switch to the known chrome window
     this.curBrowser = this.browsers[winProperties.outerId];
-
+    this.mainFrame = this.curBrowser.window;
+    this.curFrame = null;
+
+    // .. and activate the tab if it's a content browser.
     if ("tabIndex" in winProperties) {
       this.curBrowser.switchToTab(
           winProperties.tabIndex, winProperties.win, focus);
     }
   }
 };
 
 GeckoDriver.prototype.getActiveFrame = function() {