--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -157,17 +157,16 @@
 #include "nsIScriptObjectPrincipal.h"
 #include "nsIScriptSecurityManager.h"
 #include "nsIStreamConverterService.h"
 #include "nsIStringBundle.h"
 #include "nsIURI.h"
 #include "nsIURIWithPrincipal.h"
 #include "nsIURL.h"
 #include "nsIWebNavigation.h"
-#include "nsIWindowMediator.h"
 #include "nsIWordBreaker.h"
 #include "nsIXPConnect.h"
 #include "nsJSUtils.h"
 #include "nsLWBrkCIID.h"
 #include "nsNetCID.h"
 #include "nsNetUtil.h"
 #include "nsNodeInfoManager.h"
 #include "nsNullPrincipal.h"
@@ -5164,33 +5163,16 @@ nsContentUtils::RemoveScriptBlocker()
 nsIWindowProvider*
 nsContentUtils::GetWindowProviderForContentProcess()
 {
   MOZ_ASSERT(XRE_IsContentProcess());
   return ContentChild::GetSingleton();
 }
 
 /* static */
-already_AddRefed<nsPIDOMWindow>
-nsContentUtils::GetMostRecentNonPBWindow()
-{
-  nsCOMPtr<nsIWindowMediator> windowMediator =
-    do_GetService(NS_WINDOWMEDIATOR_CONTRACTID);
-  nsCOMPtr<nsIWindowMediator_44> wm = do_QueryInterface(windowMediator);
-
-  nsCOMPtr<nsIDOMWindow> window;
-  wm->GetMostRecentNonPBWindow(MOZ_UTF16("navigator:browser"),
-                               getter_AddRefs(window));
-  nsCOMPtr<nsPIDOMWindow> pwindow;
-  pwindow = do_QueryInterface(window);
-
-  return pwindow.forget();
-}
-
-/* static */
 void
 nsContentUtils::WarnScriptWasIgnored(nsIDocument* aDocument)
 {
   nsAutoString msg;
   if (aDocument) {
     nsCOMPtr<nsIURI> uri = aDocument->GetDocumentURI();
     if (uri) {
       nsCString spec;

--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -1661,21 +1661,16 @@ public:
     return sScriptBlockerCount == 0;
   }
 
   // XXXcatalinb: workaround for weird include error when trying to reference
   // ipdl types in WindowWatcher.
   static nsIWindowProvider*
   GetWindowProviderForContentProcess();
 
-  // Returns the browser window with the most recent time stamp that is
-  // not in private browsing mode.
-  static already_AddRefed<nsPIDOMWindow>
-  GetMostRecentNonPBWindow();
-
   /**
    * Call this function if !IsSafeToRunScript() and we fail to run the script
    * (rather than using AddScriptRunner as we usually do). |aDocument| is
    * optional as it is only used for showing the URL in the console.
    */
   static void WarnScriptWasIgnored(nsIDocument* aDocument);
 
   /**

--- a/dom/bindings/test/test_exception_options_from_jsimplemented.html
+++ b/dom/bindings/test/test_exception_options_from_jsimplemented.html
@@ -13,17 +13,17 @@ https://bugzilla.mozilla.org/show_bug.cg
   /** Test for Bug 1107592 **/
 
   SimpleTest.waitForExplicitFinish();
 
   function doTest() {
     var file = location.href;
     var asyncFrame;
     /* Async parent frames from pushPrefEnv don't show up in e10s.  */
-    var isE10S = !SpecialPowers.isMainProcess();
+    var isE10S = !SpecialPowers.Services.wm.getMostRecentWindow("navigator:browser");
     if (!isE10S && SpecialPowers.getBoolPref("javascript.options.asyncstack")) {
       asyncFrame = `Async*@${file}:153:1
 `;
     } else {
       asyncFrame = "";
     }
 
     var t = new TestInterfaceJS();

--- a/dom/bindings/test/test_promise_rejections_from_jsimplemented.html
+++ b/dom/bindings/test/test_promise_rejections_from_jsimplemented.html
@@ -33,17 +33,17 @@ https://bugzilla.mozilla.org/show_bug.cg
 
   function ensurePromiseFail(testNumber, value) {
     ok(false, "Test " + testNumber + " should not have a fulfilled promise");
   }
 
   function doTest() {
     var t = new TestInterfaceJS();
     /* Async parent frames from pushPrefEnv don't show up in e10s.  */
-    var isE10S = !SpecialPowers.isMainProcess();
+    var isE10S = !SpecialPowers.Services.wm.getMostRecentWindow("navigator:browser");
     var asyncStack = SpecialPowers.getBoolPref("javascript.options.asyncstack");
     var ourFile = location.href;
     var parentFrame = (asyncStack && !isE10S) ? `Async*@${ourFile}:121:1
 ` : "";
 
     Promise.all([
       t.testPromiseWithThrowingChromePromiseInit().then(
           ensurePromiseFail.bind(null, 1),

--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -143,16 +143,17 @@
 #include "nsIScriptError.h"
 #include "nsIScriptSecurityManager.h"
 #include "nsISiteSecurityService.h"
 #include "nsISpellChecker.h"
 #include "nsISupportsPrimitives.h"
 #include "nsISystemMessagesInternal.h"
 #include "nsITimer.h"
 #include "nsIURIFixup.h"
+#include "nsIWindowMediator.h"
 #include "nsIDocShellTreeOwner.h"
 #include "nsIXULWindow.h"
 #include "nsIDOMChromeWindow.h"
 #include "nsIWindowWatcher.h"
 #include "nsPIWindowWatcher.h"
 #include "nsWindowWatcher.h"
 #include "nsIXULRuntime.h"
 #include "gfxDrawable.h"
@@ -5383,16 +5384,44 @@ ContentParent::AllocPWebBrowserPersistDo
 
 bool
 ContentParent::DeallocPWebBrowserPersistDocumentParent(PWebBrowserPersistDocumentParent* aActor)
 {
   delete aActor;
   return true;
 }
 
+static already_AddRefed<nsPIDOMWindow>
+FindMostRecentOpenWindow()
+{
+    nsCOMPtr<nsIWindowMediator> windowMediator =
+        do_GetService(NS_WINDOWMEDIATOR_CONTRACTID);
+    nsCOMPtr<nsISimpleEnumerator> windowEnumerator;
+    windowMediator->GetEnumerator(MOZ_UTF16("navigator:browser"),
+                                  getter_AddRefs(windowEnumerator));
+
+    nsCOMPtr<nsPIDOMWindow> latest;
+
+    bool hasMore = false;
+    MOZ_ALWAYS_TRUE(NS_SUCCEEDED(windowEnumerator->HasMoreElements(&hasMore)));
+    while (hasMore) {
+        nsCOMPtr<nsISupports> item;
+        MOZ_ALWAYS_TRUE(NS_SUCCEEDED(windowEnumerator->GetNext(getter_AddRefs(item))));
+        nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(item);
+
+        if (window && !window->Closed()) {
+            latest = window;
+        }
+
+        MOZ_ALWAYS_TRUE(NS_SUCCEEDED(windowEnumerator->HasMoreElements(&hasMore)));
+    }
+
+    return latest.forget();
+}
+
 bool
 ContentParent::RecvCreateWindow(PBrowserParent* aThisTab,
                                 PBrowserParent* aNewTab,
                                 const uint32_t& aChromeFlags,
                                 const bool& aCalledFromJS,
                                 const bool& aPositionSpecified,
                                 const bool& aSizeSpecified,
                                 const nsCString& aURI,
@@ -5456,17 +5485,17 @@ ContentParent::RecvCreateWindow(PBrowser
     nsCOMPtr<nsIBrowserDOMWindow> browserDOMWin;
     if (thisTabParent) {
         browserDOMWin = thisTabParent->GetBrowserDOMWindow();
     }
 
     // If we haven't found a chrome window to open in, just use the most recently
     // opened one.
     if (!parent) {
-        parent = nsContentUtils::GetMostRecentNonPBWindow();
+        parent = FindMostRecentOpenWindow();
         if (NS_WARN_IF(!parent)) {
             *aResult = NS_ERROR_FAILURE;
             return true;
         }
 
         nsCOMPtr<nsIDOMChromeWindow> rootChromeWin = do_QueryInterface(parent);
         if (rootChromeWin) {
             rootChromeWin->GetBrowserDOMWindow(getter_AddRefs(browserDOMWin));

--- a/dom/workers/ServiceWorkerClients.cpp
+++ b/dom/workers/ServiceWorkerClients.cpp
@@ -610,19 +610,20 @@ private:
                            false, false, true, nullptr, nullptr,
                            getter_AddRefs(newWindow));
       nsCOMPtr<nsPIDOMWindow> pwindow = do_QueryInterface(newWindow);
       pwindow.forget(aWindow);
       return NS_OK;
     }
 
     // Find the most recent browser window and open a new tab in it.
-    nsCOMPtr<nsPIDOMWindow> browserWindow =
-      nsContentUtils::GetMostRecentNonPBWindow();
-    if (!browserWindow) {
+    nsCOMPtr<nsIDOMWindow> browserWindow;
+    rv = wm->GetMostRecentWindow(MOZ_UTF16("navigator:browser"),
+                                 getter_AddRefs(browserWindow));
+    if (NS_WARN_IF(NS_FAILED(rv)) || !browserWindow) {
       // It is possible to be running without a browser window on Mac OS, so
       // we need to open a new chrome window.
       // TODO(catalinb): open new chrome window. Bug 1218080
       return NS_ERROR_FAILURE;
     }
 
     nsCOMPtr<nsIDOMChromeWindow> chromeWin = do_QueryInterface(browserWindow);
     if (NS_WARN_IF(!chromeWin)) {

--- a/xpfe/appshell/nsIWindowMediator.idl
+++ b/xpfe/appshell/nsIWindowMediator.idl
@@ -186,20 +186,8 @@ interface nsIWindowMediator: nsISupports
     */
   void addListener(in nsIWindowMediatorListener aListener);
 
   /** Unregister a listener of window status changes.
     * @param aListener the listener to unregister
     */
   void removeListener(in nsIWindowMediatorListener aListener);
 };
-
-// XXXcatalinb: This should be merged to nsIWindowMediator. Using this
-// to avoid UUID change in aurora.
-[scriptable, uuid(88b54988-7f3c-40d2-a625-7e42a9c196c2)]
-interface nsIWindowMediator_44 : nsIWindowMediator
-{
-  /**
-   * Same as getMostRecentWindow, but ignores private browsing
-   * windows.
-   */
-  nsIDOMWindow getMostRecentNonPBWindow(in wstring aWindowType);
-};

--- a/xpfe/appshell/nsWindowMediator.cpp
+++ b/xpfe/appshell/nsWindowMediator.cpp
@@ -270,88 +270,55 @@ nsWindowMediator::GetMostRecentWindow(co
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
   NS_ENSURE_ARG_POINTER(outWindow);
   *outWindow = nullptr;
   if (!mReady)
     return NS_OK;
 
   // Find the most window with the highest time stamp that matches
   // the requested type
-  nsWindowInfo* info = MostRecentWindowInfo(inType, false);
+  nsWindowInfo *info = MostRecentWindowInfo(inType);
   if (info && info->mWindow) {
     nsCOMPtr<nsIDOMWindow> DOMWindow;
     if (NS_SUCCEEDED(GetDOMWindow(info->mWindow, DOMWindow))) {  
       *outWindow = DOMWindow;
       NS_ADDREF(*outWindow);
       return NS_OK;
     }
     return NS_ERROR_FAILURE;
   }
 
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsWindowMediator::GetMostRecentNonPBWindow(const char16_t* aType, nsIDOMWindow** aWindow)
-{
-  MOZ_RELEASE_ASSERT(NS_IsMainThread());
-  NS_ENSURE_ARG_POINTER(aWindow);
-  *aWindow = nullptr;
-
-  nsWindowInfo *info = MostRecentWindowInfo(aType, true);
-  nsCOMPtr<nsIDOMWindow> domWindow;
-  if (info && info->mWindow) {
-    GetDOMWindow(info->mWindow, domWindow);
-  }
-
-  if (!domWindow) {
-    return NS_ERROR_FAILURE;
-  }
-
-  domWindow.forget(aWindow);
-  return NS_OK;
-}
-
 nsWindowInfo*
-nsWindowMediator::MostRecentWindowInfo(const char16_t* inType, bool aSkipPrivateBrowsing)
+nsWindowMediator::MostRecentWindowInfo(const char16_t* inType)
 {
   int32_t       lastTimeStamp = -1;
   nsAutoString  typeString(inType);
   bool          allWindows = !inType || typeString.IsEmpty();
 
-  // Find the most recent window with the highest time stamp that matches
-  // the requested type and has the correct browsing mode.
-  nsWindowInfo* searchInfo = mOldestWindow;
-  nsWindowInfo* listEnd = nullptr;
-  nsWindowInfo* foundInfo = nullptr;
-  for (; searchInfo != listEnd; searchInfo = searchInfo->mYounger) {
-    listEnd = mOldestWindow;
+  // Find the most window with the highest time stamp that matches
+  // the requested type
+  nsWindowInfo *searchInfo,
+               *listEnd,
+               *foundInfo = nullptr;
 
-    if (!allWindows && !searchInfo->TypeEquals(typeString)) {
-      continue;
-    }
-    if (searchInfo->mTimeStamp < lastTimeStamp) {
-      continue;
+  searchInfo = mOldestWindow;
+  listEnd = nullptr;
+  while (searchInfo != listEnd) {
+    if ((allWindows || searchInfo->TypeEquals(typeString)) &&
+        searchInfo->mTimeStamp >= lastTimeStamp) {
+
+      foundInfo = searchInfo;
+      lastTimeStamp = searchInfo->mTimeStamp;
     }
-    if (!searchInfo->mWindow) {
-      continue;
-    }
-    if (aSkipPrivateBrowsing) {
-      nsCOMPtr<nsIDocShell> docShell;
-      searchInfo->mWindow->GetDocShell(getter_AddRefs(docShell));
-      nsCOMPtr<nsILoadContext> loadContext = do_QueryInterface(docShell);
-      if (!loadContext || loadContext->UsePrivateBrowsing()) {
-        continue;
-      }
-    }
-
-    foundInfo = searchInfo;
-    lastTimeStamp = searchInfo->mTimeStamp;
+    searchInfo = searchInfo->mYounger;
+    listEnd = mOldestWindow;
   }
-
   return foundInfo;
 }
 
 NS_IMETHODIMP
 nsWindowMediator::GetOuterWindowWithId(uint64_t aWindowID,
                                        nsIDOMWindow** aWindow)
 {
   *aWindow = nsGlobalWindow::GetOuterWindowWithId(aWindowID);
@@ -769,17 +736,16 @@ nsWindowMediator::SortZOrderBackToFront(
       scan = scan->mHigher;
     }
   } while (!finished);
 
   mSortingZOrder = false;
 }
 
 NS_IMPL_ISUPPORTS(nsWindowMediator,
-  nsIWindowMediator_44,
   nsIWindowMediator,
   nsIObserver,
   nsISupportsWeakReference)
 
 NS_IMETHODIMP
 nsWindowMediator::AddListener(nsIWindowMediatorListener* aListener)
 {
   NS_ENSURE_ARG_POINTER(aListener);

--- a/xpfe/appshell/nsWindowMediator.h
+++ b/xpfe/appshell/nsWindowMediator.h
@@ -20,17 +20,17 @@ class nsASDOMWindowEarlyToLateEnumerator
 class nsASDOMWindowFrontToBackEnumerator;
 class nsASXULWindowFrontToBackEnumerator;
 class nsASDOMWindowBackToFrontEnumerator;
 class nsASXULWindowBackToFrontEnumerator;
 class nsIWindowMediatorListener;
 struct nsWindowInfo;
 
 class nsWindowMediator :
-  public nsIWindowMediator_44,
+  public nsIWindowMediator,
   public nsIObserver,
   public nsSupportsWeakReference
 {
 friend class nsAppShellWindowEnumerator;
 friend class nsASXULWindowEarlyToLateEnumerator;
 friend class nsASDOMWindowEarlyToLateEnumerator;
 friend class nsASDOMWindowFrontToBackEnumerator;
 friend class nsASXULWindowFrontToBackEnumerator;
@@ -42,27 +42,25 @@ protected:
 
 public:
   nsWindowMediator();
 
   nsresult Init();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSIWINDOWMEDIATOR
-  NS_DECL_NSIWINDOWMEDIATOR_44
   NS_DECL_NSIOBSERVER
 
   static nsresult GetDOMWindow(nsIXULWindow* inWindow,
                                nsCOMPtr<nsIDOMWindow>& outDOMWindow);
 
 private:
   int32_t AddEnumerator(nsAppShellWindowEnumerator* inEnumerator);
   int32_t RemoveEnumerator(nsAppShellWindowEnumerator* inEnumerator);
-  nsWindowInfo* MostRecentWindowInfo(const char16_t* inType,
-                                     bool aSkipPrivateBrowsing = false);
+  nsWindowInfo *MostRecentWindowInfo(const char16_t* inType);
 
   nsresult      UnregisterWindow(nsWindowInfo *inInfo);
   nsWindowInfo *GetInfoFor(nsIXULWindow *aWindow);
   nsWindowInfo *GetInfoFor(nsIWidget *aWindow);
   void          SortZOrderFrontToBack();
   void          SortZOrderBackToFront();
 
   nsTArray<nsAppShellWindowEnumerator*> mEnumeratorList;