diff --git a/xpfe/appshell/src/nsWebShellWindow.cpp b/xpfe/appshell/src/nsWebShellWindow.cpp
index dfe10a111581..8870259ff0b4 100644
--- a/xpfe/appshell/src/nsWebShellWindow.cpp
+++ b/xpfe/appshell/src/nsWebShellWindow.cpp
@@ -92,12 +92,6 @@ nsWebShellWindow::nsWebShellWindow(PRUint32 aChromeFlags)
 
 nsWebShellWindow::~nsWebShellWindow()
 {
-  if (mWindow) {
-    mWindow->SetClientData(0);
-    mWindow->Destroy();
-    mWindow = nullptr; // Force release here.
-  }
-
   MutexAutoLock lock(mSPTimerLock);
   if (mSPTimer)
     mSPTimer->Cancel();
diff --git a/xpfe/appshell/src/nsWindowMediator.cpp b/xpfe/appshell/src/nsWindowMediator.cpp
index c32f8a18bebd..f0a40f767992 100644
--- a/xpfe/appshell/src/nsWindowMediator.cpp
+++ b/xpfe/appshell/src/nsWindowMediator.cpp
@@ -757,9 +757,19 @@ nsWindowMediator::Observe(nsISupports* aSubject,
                           const PRUnichar* aData)
 {
   if (!strcmp(aTopic, "xpcom-shutdown") && mReady) {
-    MutexAutoLock lock(mListLock);
-    while (mOldestWindow)
-      UnregisterWindow(mOldestWindow);
+    // Unregistering a window may cause its destructor to run, causing it to
+    // call into the window mediator, try to acquire mListLock, and deadlock.
+    // Our solution is to hold strong refs to all windows until we release
+    // mListLock.
+    nsTArray<nsCOMPtr<nsIXULWindow> > windows;
+
+    {
+      MutexAutoLock lock(mListLock);
+      while (mOldestWindow) {
+        windows.AppendElement(mOldestWindow->mWindow);
+        UnregisterWindow(mOldestWindow);
+      }
+    }
     mReady = false;
   }
   return NS_OK;