--- a/accessible/tests/mochitest/test_nsIAccessibleDocument.html
+++ b/accessible/tests/mochitest/test_nsIAccessibleDocument.html
@@ -62,17 +62,17 @@ https://bugzilla.mozilla.org/show_bug.cg
           domDoc = docAcc.DOMDocument.QueryInterface(nsIDOMDocument);
         } catch (e) {}
         ok(domDoc, "no nsIDOMDocument for this doc accessible!");
         is(domDoc, document, "Document nodes do not match!");
 
         // Test for correct nsIDOMWindow retrieval.
         var domWindow = null;
         try {
-          domWindow = docAcc.window.QueryInterface(nsIDOMWindow);
+          domWindow = docAcc.window;
         } catch (e) {}
         ok(domWindow, "no nsIDOMWindow for this doc accessible!");
         is(domWindow, window, "Window nodes do not match!");
       }
 
       SimpleTest.finish();
     }
 

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -6841,17 +6841,17 @@ var CanvasPermissionPromptHelper = {
   // aData is an URL string.
   observe(aSubject, aTopic, aData) {
     if (aTopic != this._permissionsPrompt) {
       return;
     }
 
     let browser;
     if (aSubject instanceof Ci.nsIDOMWindow) {
-      let contentWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      let contentWindow = aSubject;
       browser = gBrowser.getBrowserForContentWindow(contentWindow);
     } else {
       browser = aSubject.QueryInterface(Ci.nsIBrowser);
     }
 
     let uri = Services.io.newURI(aData);
     if (gBrowser.selectedBrowser !== browser) {
       // Must belong to some other window.
@@ -7217,17 +7217,16 @@ function BrowserOpenAddonsMgr(aView) {
                       "tab";
     openTrustedLinkIn("about:addons", whereToOpen);
 
     Services.obs.addObserver(function observer(aSubject, aTopic, aData) {
       Services.obs.removeObserver(observer, aTopic);
       if (aView) {
         aSubject.loadView(aView);
       }
-      aSubject.QueryInterface(Ci.nsIDOMWindow);
       aSubject.focus();
       resolve(aSubject);
     }, "EM-loaded");
   });
 }
 
 function AddKeywordForSearchField() {
   let mm = gBrowser.selectedBrowser.messageManager;

--- a/browser/base/content/test/general/browser_newwindow_focus.js
+++ b/browser/base/content/test/general/browser_newwindow_focus.js
@@ -14,20 +14,19 @@ const PAGE = `data:text/html,<a id="targ
  * because by the time the "then" on the Promise runs,
  * DOMContentLoaded and load may have already run in the new
  * window. However, we want to be very explicit about what
  * events we're waiting for, and not rely on a quirk of our
  * Promises infrastructure.
  */
 function promiseNewWindow() {
   return new Promise((resolve) => {
-    let observer = (subject, topic, data) => {
+    let observer = (win, topic, data) => {
       if (topic == "domwindowopened") {
         Services.ww.unregisterNotification(observer);
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
         win.addEventListener("load", function() {
           resolve(win);
         }, {once: true});
       }
     };
 
     Services.ww.registerNotification(observer);
   });

--- a/browser/base/content/test/sanitize/browser_sanitizeDialog.js
+++ b/browser/base/content/test/sanitize/browser_sanitizeDialog.js
@@ -777,24 +777,23 @@ WindowHelper.prototype = {
    * when done so that the tests may continue.  Set this.onunload to a function
    * to execute onunload.  this.onunload is optional. If it returns true, the
    * caller is expected to call promiseAsyncUpdates at some point; if false is
    * returned, promiseAsyncUpdates is called automatically.
    */
   open() {
     let wh = this;
 
-    function windowObserver(aSubject, aTopic, aData) {
+    function windowObserver(win, aTopic, aData) {
       if (aTopic != "domwindowopened")
         return;
 
       Services.ww.unregisterNotification(windowObserver);
 
       var loaded = false;
-      let win = aSubject.QueryInterface(Ci.nsIDOMWindow);
 
       win.addEventListener("load", function onload(event) {
         if (win.name !== "SanitizeDialog")
           return;
 
         wh.win = win;
         loaded = true;
         executeSoon(() => wh.onload());

--- a/browser/base/content/test/webrtc/head.js
+++ b/browser/base/content/test/webrtc/head.js
@@ -46,17 +46,16 @@ function promiseWaitForCondition(aCondit
  *        The url of the window.
  * @return {Promise} resolved when the window exists.
  * @resolves to the window
  */
 function promiseWindow(url) {
   info("expecting a " + url + " window");
   return new Promise(resolve => {
     Services.obs.addObserver(function obs(win) {
-      win.QueryInterface(Ci.nsIDOMWindow);
       win.addEventListener("load", function() {
         if (win.location.href !== url) {
           info("ignoring a window with this url: " + win.location.href);
           return;
         }
 
         Services.obs.removeObserver(obs, "domwindowopened");
         executeSoon(() => resolve(win));

--- a/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
+++ b/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
@@ -15,17 +15,17 @@ add_task(async function() {
   info("Menu panel was opened");
 
   let windowWasHandled = false;
   let privateWindow = null;
 
   let observerWindowOpened = {
     observe(aSubject, aTopic, aData) {
       if (aTopic == "domwindowopened") {
-        privateWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+        privateWindow = aSubject;
         privateWindow.addEventListener("load", function() {
           is(privateWindow.location.href, "chrome://browser/content/browser.xul",
              "A new browser window was opened");
           ok(PrivateBrowsingUtils.isWindowPrivate(privateWindow), "Window is private");
           windowWasHandled = true;
         }, {once: true});
       }
     }

--- a/browser/components/customizableui/test/browser_947914_button_newWindow.js
+++ b/browser/components/customizableui/test/browser_947914_button_newWindow.js
@@ -15,17 +15,17 @@ add_task(async function() {
   info("Menu panel was opened");
 
   let windowWasHandled = false;
   let newWindow = null;
 
   let observerWindowOpened = {
     observe(aSubject, aTopic, aData) {
       if (aTopic == "domwindowopened") {
-        newWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+        newWindow = aSubject;
         newWindow.addEventListener("load", function() {
           is(newWindow.location.href, "chrome://browser/content/browser.xul",
              "A new browser window was opened");
           ok(!PrivateBrowsingUtils.isWindowPrivate(newWindow), "Window is not private");
           windowWasHandled = true;
         }, {once: true});
       }
     }

--- a/browser/components/places/tests/browser/browser_bookmarksProperties.js
+++ b/browser/components/places/tests/browser/browser_bookmarksProperties.js
@@ -399,21 +399,20 @@ function open_properties_dialog(test) {
     await BrowserTestUtils.waitForCondition(() => tree, "Sidebar tree has been loaded");
 
     // Ask current test to select the node to edit.
     test.selectNode(tree);
     Assert.ok(tree.selectedNode,
        "We have a places node selected: " + tree.selectedNode.title);
 
     // Wait for the Properties dialog.
-    function windowObserver(aSubject, aTopic, aData) {
+    function windowObserver(observerWindow, aTopic, aData) {
       if (aTopic != "domwindowopened")
         return;
       Services.ww.unregisterNotification(windowObserver);
-      let observerWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
       waitForFocus(async () => {
         // Ensure overlay is loaded
         await BrowserTestUtils.waitForCondition(
           () => observerWindow.gEditItemOverlay.initialized, "EditItemOverlay is initialized");
         test.window = observerWindow;
         try {
           executeSoon(resolve);
         } catch (ex) {

--- a/browser/components/places/tests/browser/head.js
+++ b/browser/components/places/tests/browser/head.js
@@ -164,17 +164,17 @@ function isToolbarVisible(aToolbar) {
  */
 var withBookmarksDialog = async function(autoCancel, openFn, taskFn, closeFn,
                                          dialogUrl = "chrome://browser/content/places/bookmarkProperties",
                                          skipOverlayWait = false) {
   let closed = false;
   let dialogPromise = new Promise(resolve => {
     Services.ww.registerNotification(function winObserver(subject, topic, data) {
       if (topic == "domwindowopened") {
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
+        let win = subject;
         win.addEventListener("load", function() {
           ok(win.location.href.startsWith(dialogUrl),
              "The bookmark properties dialog is open: " + win.location.href);
           // This is needed for the overlay.
           waitForFocus(() => {
             resolve(win);
           }, win);
         }, {once: true});

--- a/browser/components/sessionstore/test/browser_586068-browser_state_interrupted.js
+++ b/browser/components/sessionstore/test/browser_586068-browser_state_interrupted.js
@@ -89,17 +89,17 @@ add_task(async function test() {
       gProgressListener.unsetCallback();
       resolve();
     });
   });
 
   // We also want to catch the extra windows (there should be 2), so we need to observe domwindowopened
   Services.ww.registerNotification(function observer(aSubject, aTopic, aData) {
     if (aTopic == "domwindowopened") {
-      let win = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      let win = aSubject;
       win.addEventListener("load", function() {
         Services.ww.unregisterNotification(observer);
         win.gBrowser.addTabsProgressListener(gProgressListener);
       }, {once: true});
     }
   });
 
   let backupState = ss.getBrowserState();

--- a/browser/components/sessionstore/test/browser_586068-multi_window.js
+++ b/browser/components/sessionstore/test/browser_586068-multi_window.js
@@ -46,17 +46,17 @@ add_task(async function test() {
         resolve();
       }
     });
   });
 
   // We also want to catch the 2nd window, so we need to observe domwindowopened
   Services.ww.registerNotification(function observer(aSubject, aTopic, aData) {
     if (aTopic == "domwindowopened") {
-      let win = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      let win = aSubject;
       win.addEventListener("load", function() {
         Services.ww.unregisterNotification(observer);
         win.gBrowser.addTabsProgressListener(gProgressListener);
       }, {once: true});
     }
   });
 
   let backupState = ss.getBrowserState();

--- a/browser/components/sessionstore/test/browser_615394-SSWindowState_events_setBrowserState.js
+++ b/browser/components/sessionstore/test/browser_615394-SSWindowState_events_setBrowserState.js
@@ -40,17 +40,17 @@ function test() {
 
   // waitForBrowserState does it's own observing for windows, but doesn't attach
   // the listeners we want here, so do it ourselves.
   let newWindow;
   function windowObserver(aSubject, aTopic, aData) {
     if (aTopic == "domwindowopened") {
       Services.ww.unregisterNotification(windowObserver);
 
-      newWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      newWindow = aSubject;
       newWindow.addEventListener("load", function() {
         windowEvents[getOuterWindowID(newWindow)] = { busyEventCount: 0, readyEventCount: 0 };
 
         newWindow.addEventListener("SSWindowStateBusy", onSSWindowStateBusy);
         newWindow.addEventListener("SSWindowStateReady", onSSWindowStateReady);
       }, {once: true});
     }
   }

--- a/browser/components/sessionstore/test/browser_615394-SSWindowState_events_undoCloseWindow.js
+++ b/browser/components/sessionstore/test/browser_615394-SSWindowState_events_undoCloseWindow.js
@@ -26,17 +26,17 @@ const lameMultiWindowState = { windows: 
 function test() {
   /** Test for Bug 615394 - Session Restore should notify when it is beginning and ending a restore **/
   waitForExplicitFinish();
 
   let newWindow, reopenedWindow;
 
   function firstWindowObserver(aSubject, aTopic, aData) {
     if (aTopic == "domwindowopened") {
-      newWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      newWindow = aSubject;
       Services.ww.unregisterNotification(firstWindowObserver);
     }
   }
   Services.ww.registerNotification(firstWindowObserver);
 
   waitForBrowserState(lameMultiWindowState, function() {
     // Close the window which isn't window
     BrowserTestUtils.closeWindow(newWindow).then(() => {

--- a/browser/components/sessionstore/test/head.js
+++ b/browser/components/sessionstore/test/head.js
@@ -118,17 +118,17 @@ function waitForBrowserState(aState, aSe
       executeSoon(aSetStateCallback);
     }
   }
 
   // Used to add our listener to further windows so we can catch SSTabRestored
   // coming from them when creating a multi-window state.
   function windowObserver(aSubject, aTopic, aData) {
     if (aTopic == "domwindowopened") {
-      let newWindow = aSubject.QueryInterface(Ci.nsIDOMWindow);
+      let newWindow = aSubject;
       newWindow.addEventListener("load", function() {
         if (++windowsOpen == expectedWindows) {
           Services.ww.unregisterNotification(windowObserver);
           windowObserving = false;
         }
 
         // Track this window so we can remove the progress listener later
         windows.push(newWindow);

--- a/browser/components/tests/browser/browser_bug538331.js
+++ b/browser/components/tests/browser/browser_bug538331.js
@@ -152,22 +152,21 @@ var gWindowCatcher = {
       this.finish(this.finishFunc);
     }
   },
 
   windowLoad(win) {
     executeSoon(this.closeWindow.bind(this, win));
   },
 
-  observe(subject, topic, data) {
+  observe(win, topic, data) {
     if (topic != "domwindowopened")
       return;
 
     this.windowsOpen++;
-    let win = subject.QueryInterface(Ci.nsIDOMWindow);
     info("window catcher caught window opening: " + win.document.documentURI);
     win.addEventListener("load", function() {
       gWindowCatcher.windowLoad(win);
     }, {once: true});
   }
 };
 
 function finish_test() {

--- a/browser/extensions/formautofill/bootstrap.js
+++ b/browser/extensions/formautofill/bootstrap.js
@@ -132,18 +132,17 @@ function startup(data) {
 function shutdown() {
   resProto.setSubstitution(RESOURCE_HOST, null);
 
   Services.mm.removeMessageListener("FormAutoComplete:MaybeOpenPopup", onMaybeOpenPopup);
 
   let enumerator = Services.wm.getEnumerator("navigator:browser");
   while (enumerator.hasMoreElements()) {
     let win = enumerator.getNext();
-    let domWindow = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindow);
-    let cachedStyleSheets = CACHED_STYLESHEETS.get(domWindow);
+    let cachedStyleSheets = CACHED_STYLESHEETS.get(win);
 
     if (!cachedStyleSheets) {
       continue;
     }
 
     while (cachedStyleSheets.length !== 0) {
       cachedStyleSheets.pop().remove();
     }

--- a/browser/modules/ProcessHangMonitor.jsm
+++ b/browser/modules/ProcessHangMonitor.jsm
@@ -238,27 +238,27 @@ var ProcessHangMonitor = {
       case "clear-hang-report": {
         this.clearHang(subject.QueryInterface(Ci.nsIHangReport));
         break;
       }
 
       case "domwindowopened": {
         // Install event listeners on the new window in case one of
         // its tabs is already hung.
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
+        let win = subject;
         let listener = (ev) => {
           win.removeEventListener("load", listener, true);
           this.updateWindows();
         };
         win.addEventListener("load", listener, true);
         break;
       }
 
       case "domwindowclosed": {
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
+        let win = subject;
         this.onWindowClosed(win);
         break;
       }
     }
   },
 
   /**
    * Called early on in the shutdown sequence. We take this opportunity to

--- a/devtools/client/scratchpad/test/browser_scratchpad_sessions.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_sessions.js
@@ -36,17 +36,17 @@ function test() {
 
   Services.ww.registerNotification(windowObserver);
 
   SessionStore.setBrowserState(JSON.stringify(testState));
 }
 
 function windowObserver(subject, topic, data) {
   if (topic == "domwindowopened") {
-    let win = subject.QueryInterface(Ci.nsIDOMWindow);
+    let win = subject;
     win.addEventListener("load", function() {
       if (win.Scratchpad) {
         win.Scratchpad.addObserver({
           onReady() {
             win.Scratchpad.removeObserver(this);
 
             let state = win.Scratchpad.getState();
             BrowserTestUtils.closeWindow(win).then(() => {

--- a/devtools/client/shared/test/browser_telemetry_button_scratchpad.js
+++ b/devtools/client/shared/test/browser_telemetry_button_scratchpad.js
@@ -36,17 +36,17 @@ add_task(async function() {
 function trackScratchpadWindows() {
   info("register the window observer to track when scratchpad windows open");
 
   let numScratchpads = 0;
 
   return new Promise(resolve => {
     Services.ww.registerNotification(function observer(subject, topic) {
       if (topic == "domwindowopened") {
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
+        let win = subject;
         win.addEventListener("load", function() {
           if (win.Scratchpad) {
             win.Scratchpad.addObserver({
               onReady: function() {
                 win.Scratchpad.removeObserver(this);
                 numScratchpads++;
                 win.close();
 

--- a/devtools/server/actors/child-process.js
+++ b/devtools/server/actors/child-process.js
@@ -28,17 +28,17 @@ function ChildProcessActor(connection) {
     shouldAddNewGlobalAsDebuggee: global => true
   });
 
   let sandboxPrototype = {
     get tabs() {
       let tabs = [];
       let windowEnumerator = Services.ww.getWindowEnumerator();
       while (windowEnumerator.hasMoreElements()) {
-        let window = windowEnumerator.getNext().QueryInterface(Ci.nsIDOMWindow);
+        let window = windowEnumerator.getNext();
         let tabChildGlobal = window.QueryInterface(Ci.nsIInterfaceRequestor)
                                    .getInterface(Ci.nsIDocShell)
                                    .sameTypeRootTreeItem
                                    .QueryInterface(Ci.nsIInterfaceRequestor)
                                    .getInterface(Ci.nsIContentFrameMessageManager);
         tabs.push(tabChildGlobal);
       }
       return tabs;

--- a/devtools/server/actors/webbrowser.js
+++ b/devtools/server/actors/webbrowser.js
@@ -648,18 +648,16 @@ DevToolsUtils.makeInfallible(function(wi
   });
 
   /*
    * You can hardly do anything at all with a XUL window at this point; it
    * doesn't even have its document yet. Wait until its document has
    * loaded, and then see what we've got. This also avoids
    * nsIWindowMediator enumeration from within listeners (bug 873589).
    */
-  window = window.QueryInterface(Ci.nsIInterfaceRequestor)
-                 .getInterface(Ci.nsIDOMWindow);
 
   window.addEventListener("load", handleLoad);
 }, "BrowserTabList.prototype.onOpenWindow");
 
 BrowserTabList.prototype.onCloseWindow =
 DevToolsUtils.makeInfallible(function(window) {
   window = window.QueryInterface(Ci.nsIInterfaceRequestor)
                    .getInterface(Ci.nsIDOMWindow);

--- a/layout/tools/reftest/bootstrap.js
+++ b/layout/tools/reftest/bootstrap.js
@@ -11,17 +11,17 @@ var WindowListener = {
   onOpenWindow: function(win) {
     Services.wm.removeListener(WindowListener);
 
     win = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindow);
     win.addEventListener("load", function listener() {
       // Load into any existing windows.
       let windows = Services.wm.getEnumerator("navigator:browser");
       while (windows.hasMoreElements()) {
-        win = windows.getNext().QueryInterface(Ci.nsIDOMWindow);
+        win = windows.getNext();
         break;
       }
 
       ChromeUtils.import("chrome://reftest/content/reftest.jsm");
       win.addEventListener("pageshow", function() {
         // Add setTimeout here because windows.innerWidth/Height are not set yet.
         win.setTimeout(function() {OnRefTestLoad(win);}, 0);
       }, {once: true});

--- a/mobile/android/modules/WebrtcUI.jsm
+++ b/mobile/android/modules/WebrtcUI.jsm
@@ -92,17 +92,17 @@ var WebrtcUI = {
         when: null, // hide the date row
         light: [0xFF9500FF, 1000, 1000],
         ongoing: true
       };
 
       let cameraActive = false;
       let audioActive = false;
       for (let i = 0; i < count; i++) {
-        let win = windows.queryElementAt(i, Ci.nsIDOMWindow);
+        let win = windows.queryElementAt(i, Ci.nsISupports);
         let hasCamera = {};
         let hasMicrophone = {};
         MediaManagerService.mediaCaptureWindowState(win, hasCamera, hasMicrophone);
         if (hasCamera.value != MediaManagerService.STATE_NOCAPTURE) cameraActive = true;
         if (hasMicrophone.value != MediaManagerService.STATE_NOCAPTURE) audioActive = true;
       }
 
       if (cameraActive && audioActive) {

--- a/mobile/android/modules/geckoview/GeckoViewUtils.jsm
+++ b/mobile/android/modules/geckoview/GeckoViewUtils.jsm
@@ -298,17 +298,17 @@ var GeckoViewUtils = {
     let win = Services.focus.activeWindow;
     let dispatcher = this.getDispatcherForWindow(win);
     if (dispatcher) {
       return [dispatcher, win];
     }
 
     let iter = Services.wm.getEnumerator(/* windowType */ null);
     while (iter.hasMoreElements()) {
-      win = iter.getNext().QueryInterface(Ci.nsIDOMWindow);
+      win = iter.getNext();
       dispatcher = this.getDispatcherForWindow(win);
       if (dispatcher) {
         return [dispatcher, win];
       }
     }
     return [null, null];
   },
 

--- a/mobile/android/tests/browser/chrome/test_awsy_lite.html
+++ b/mobile/android/tests/browser/chrome/test_awsy_lite.html
@@ -119,17 +119,17 @@
 
   function startup() {
     var enumerator = Services.wm.getEnumerator("navigator:browser");
     while (enumerator.hasMoreElements()) {
         // potential race condition here - the window may not be ready yet at
         // this point, so ideally we would test for that. but i can't find a
         // property that reflects whether or not UIReady has been fired, so
         // for now just assume the window is ready
-        attachTo(enumerator.getNext().QueryInterface(Ci.nsIDOMWindow));
+        attachTo(enumerator.getNext());
     }
     Services.wm.addListener(browserListener);
   }
 
   function startTest() {
     checkpoint("Fresh start");
     setTimeout(settle, 30000);
   }

--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
@@ -613,17 +613,17 @@ var BrowserTestUtils = {
    * @return {Promise}
    *         A Promise which resolves when a "domwindowopened" notification
    *         has been fired by the window watcher.
    */
   domWindowOpened(win, checkFn) {
     return new Promise(resolve => {
       async function observer(subject, topic, data) {
         if (topic == "domwindowopened" && (!win || subject === win)) {
-          let observedWindow = subject.QueryInterface(Ci.nsIDOMWindow);
+          let observedWindow = subject;
           if (checkFn && !await checkFn(observedWindow)) {
             return;
           }
           Services.ww.unregisterNotification(observer);
           resolve(observedWindow);
         }
       }
       Services.ww.registerNotification(observer);
@@ -639,17 +639,17 @@ var BrowserTestUtils = {
    *         A Promise which resolves when a "domwindowclosed" notification
    *         has been fired by the window watcher.
    */
   domWindowClosed(win) {
     return new Promise((resolve) => {
       function observer(subject, topic, data) {
         if (topic == "domwindowclosed" && (!win || subject === win)) {
           Services.ww.unregisterNotification(observer);
-          resolve(subject.QueryInterface(Ci.nsIDOMWindow));
+          resolve(subject);
         }
       }
       Services.ww.registerNotification(observer);
     });
   },
 
   /**
    * @param {Object} options

--- a/testing/mochitest/bootstrap.js
+++ b/testing/mochitest/bootstrap.js
@@ -104,17 +104,17 @@ function startup(data, reason) {
     win.addEventListener("mochitest-load", loadMochitest);
   }
 }
 
 function shutdown(data, reason) {
   if (AppConstants.platform != "android") {
     let windows = Services.wm.getEnumerator("navigator:browser");
     while (windows.hasMoreElements()) {
-      let win = windows.getNext().QueryInterface(Ci.nsIDOMWindow);
+      let win = windows.getNext();
       WindowListener.tearDownWindow(win);
     }
 
     Services.wm.removeListener(WindowListener);
   }
 }
 
 function install(data, reason) {}

--- a/toolkit/components/alerts/test/test_alerts_requireinteraction.html
+++ b/toolkit/components/alerts/test/test_alerts_requireinteraction.html
@@ -19,22 +19,21 @@ const chromeScript = SpecialPowers.loadC
   ChromeUtils.import("resource://gre/modules/Timer.jsm");
 
   addMessageListener("waitForXULAlert", function() {
     var timer = setTimeout(function() {
       Services.ww.unregisterNotification(windowObserver);
       sendAsyncMessage("waitForXULAlert", false);
     }, 2000);
 
-    var windowObserver = function(aSubject, aTopic, aData) {
+    var windowObserver = function(win, aTopic, aData) {
       if (aTopic != "domwindowopened") {
         return;
       }
 
-      var win = aSubject.QueryInterface(Ci.nsIDOMWindow);
       win.addEventListener("load", function() {
         let windowType = win.document.documentElement.getAttribute("windowtype");
         if (windowType == "alert:alert") {
           clearTimeout(timer);
           Services.ww.unregisterNotification(windowObserver);
 
           sendAsyncMessage("waitForXULAlert", true);
         }

--- a/toolkit/components/alerts/test/test_multiple_alerts.html
+++ b/toolkit/components/alerts/test/test_multiple_alerts.html
@@ -22,27 +22,26 @@ const chromeScript = SpecialPowers.loadC
                          .getService(Ci.nsIAlertsService);
 
   addMessageListener("waitForPosition", function() {
     var timer = setTimeout(function() {
       Services.ww.unregisterNotification(windowObserver);
       sendAsyncMessage("waitedForPosition", null);
     }, 2000);
 
-    var windowObserver = function(aSubject, aTopic, aData) {
+    var windowObserver = function(win, aTopic, aData) {
       if (aTopic != "domwindowopened") {
         return;
       }
 
       // Alerts are implemented using XUL.
       clearTimeout(timer);
 
       Services.ww.unregisterNotification(windowObserver);
 
-      var win = aSubject.QueryInterface(Ci.nsIDOMWindow);
       win.addEventListener("pageshow", function() {
         var x = win.screenX;
         var y = win.screenY;
 
         win.addEventListener("pagehide", function() {
           sendAsyncMessage("waitedForPosition", { x, y });
         }, {once: true});
 

--- a/toolkit/components/viewsource/content/viewSourceUtils.js
+++ b/toolkit/components/viewsource/content/viewSourceUtils.js
@@ -56,24 +56,22 @@ var gViewSourceUtils = {
     // Try existing browsers first.
     let browserWin = Services.wm.getMostRecentWindow("navigator:browser");
     if (browserWin && browserWin.BrowserViewSourceOfDocument) {
       browserWin.BrowserViewSourceOfDocument(aArgs);
       return;
     }
     // No browser window created yet, try to create one.
     let utils = this;
-    Services.ww.registerNotification(function onOpen(subj, topic) {
-      if (subj.document.documentURI !== "about:blank" ||
+    Services.ww.registerNotification(function onOpen(win, topic) {
+      if (win.document.documentURI !== "about:blank" ||
           topic !== "domwindowopened") {
         return;
       }
       Services.ww.unregisterNotification(onOpen);
-      let win = subj.QueryInterface(Ci.nsIInterfaceRequestor)
-                    .getInterface(Ci.nsIDOMWindow);
       win.addEventListener("load", () => {
         aArgs.viewSourceBrowser = win.gBrowser.selectedTab.linkedBrowser;
         utils.viewSourceInBrowser(aArgs);
       }, { once: true });
     });
     window.top.openWebLinkIn("about:blank", "current");
   },
 

--- a/toolkit/content/tests/browser/browser_f7_caret_browsing.js
+++ b/toolkit/content/tests/browser/browser_f7_caret_browsing.js
@@ -26,17 +26,17 @@ registerCleanupFunction(function() {
 // then "stop" waiting so that when we next *do* want it to open, our "old"
 // listener doesn't fire and do things we don't want (like close the window...).
 let gCaretPromptOpeningObserver;
 function promiseCaretPromptOpened() {
   return new Promise(resolve => {
     function observer(subject, topic, data) {
       if (topic == "domwindowopened") {
         Services.ww.unregisterNotification(observer);
-        let win = subject.QueryInterface(Ci.nsIDOMWindow);
+        let win = subject;
         BrowserTestUtils.waitForEvent(win, "load", false, e => e.target.location.href != "about:blank").then(() => resolve(win));
         gCaretPromptOpeningObserver = null;
       }
     }
     Services.ww.registerNotification(observer);
     gCaretPromptOpeningObserver = observer;
   });
 }

--- a/toolkit/mozapps/extensions/test/browser/browser_bug523784.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_bug523784.js
@@ -16,23 +16,22 @@ var args = {
     blocked: false,
     url: "http://example.com/bug523784_1",
   }],
 };
 
 function test() {
   waitForExplicitFinish();
 
-  let windowObserver = function(aSubject, aTopic, aData) {
+  let windowObserver = function(win, aTopic, aData) {
     if (aTopic != "domwindowopened")
       return;
 
     Services.ww.unregisterNotification(windowObserver);
 
-    let win = aSubject.QueryInterface(Ci.nsIDOMWindow);
     win.addEventListener("load", function() {
       executeSoon(() => bug523784_test1(win));
     }, {once: true});
   };
   Services.ww.registerNotification(windowObserver);
 
   args.wrappedJSObject = args;
   Services.ww.openWindow(null, URI_BLOCKLIST_DIALOG, "",
@@ -66,22 +65,21 @@ function bug523784_test1(win) {
     executeSoon(bug523784_test2);
   };
   Services.ww.registerNotification(windowObserver);
 
   cancelButton.doCommand();
 }
 
 function bug523784_test2(win) {
-  let windowObserver = function(aSubject, aTopic, aData) {
+  let windowObserver = function(win, aTopic, aData) {
     if (aTopic != "domwindowopened")
       return;
 
     Services.ww.unregisterNotification(windowObserver);
-    let win = aSubject.QueryInterface(Ci.nsIDOMWindow);
     win.addEventListener("load", function() {
       executeSoon(function() {
       let moreInfoLink = win.document.getElementById("moreInfo");
       let cancelButton = win.document.documentElement.getButton("cancel");
       is(moreInfoLink.getAttribute("href"),
          Services.urlFormatter.formatURLPref("extensions.blocklist.detailsURL"),
          "More Info link should link to the general blocklist page.");
       cancelButton.doCommand();

--- a/toolkit/mozapps/extensions/test/browser/head.js
+++ b/toolkit/mozapps/extensions/test/browser/head.js
@@ -93,17 +93,17 @@ function promiseFocus(window) {
   return new Promise(resolve => waitForFocus(resolve, window));
 }
 
 // Helper to register test failures and close windows if any are left open
 function checkOpenWindows(aWindowID) {
   let windows = Services.wm.getEnumerator(aWindowID);
   let found = false;
   while (windows.hasMoreElements()) {
-    let win = windows.getNext().QueryInterface(Ci.nsIDOMWindow);
+    let win = windows.getNext();
     if (!win.closed) {
       found = true;
       win.close();
     }
   }
   if (found)
     ok(false, "Found unexpected " + aWindowID + " window still open");
 }

--- a/widget/tests/test_imestate.html
+++ b/widget/tests/test_imestate.html
@@ -1321,17 +1321,17 @@ function runTestPasswordFieldOnDialog()
         return this;
       }
     },
 
     observe: function (subject, topic, data)
     {
       if (topic === "domwindowopened") {
         ok(true, "dialog window is created");
-        dialog = subject.QueryInterface(Ci.nsIDOMWindow);
+        dialog = subject;
         dialog.addEventListener("load", onPasswordDialogLoad);
       }
     }
   };
 
   var observer = new WindowObserver();
   var arg1 = new Object(), arg2 = new Object();
   Cc["@mozilla.org/embedcomp/prompt-service;1"].