--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -967,18 +967,17 @@ Navigator::GetMozPower(nsIDOMMozPowerMan
   if (!mPowerManager) {
     nsCOMPtr<nsPIDOMWindow> win = do_QueryReferent(mWindow);
     NS_ENSURE_TRUE(win, NS_OK);
 
     mPowerManager = new power::PowerManager();
     mPowerManager->Init(win);
   }
 
-  nsCOMPtr<nsIDOMMozPowerManager> power =
-    do_QueryInterface(NS_ISUPPORTS_CAST(nsIDOMMozPowerManager*, mPowerManager));
+  nsCOMPtr<nsIDOMMozPowerManager> power(mPowerManager);
   power.forget(aPower);
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 Navigator::RequestWakeLock(const nsAString &aTopic, nsIDOMMozWakeLock **aWakeLock)
 {

--- a/dom/power/PowerManagerService.cpp
+++ b/dom/power/PowerManagerService.cpp
@@ -158,18 +158,17 @@ NS_IMETHODIMP
 PowerManagerService::NewWakeLock(const nsAString &aTopic,
                                  nsIDOMWindow *aWindow,
                                  nsIDOMMozWakeLock **aWakeLock)
 {
   nsRefPtr<WakeLock> wakelock = new WakeLock();
   nsresult rv = wakelock->Init(aTopic, aWindow);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  nsCOMPtr<nsIDOMMozWakeLock> wl =
-    do_QueryInterface(NS_ISUPPORTS_CAST(nsIDOMMozWakeLock*, wakelock));
+  nsCOMPtr<nsIDOMMozWakeLock> wl(wakelock);
   wl.forget(aWakeLock);
 
   return NS_OK;
 }
 
 } // power
 } // dom
 } // mozilla