--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -254,17 +254,17 @@ interface nsIDOMWindowUtils : nsISupport
    * Whether the next paint should be flagged as the first paint for a document.
    * This gives a way to track the next paint that occurs after the flag is
    * set. The flag gets cleared after the next paint.
    *
    * Can only be accessed with chrome privileges.
    */
   attribute boolean isFirstPaint;
 
-  void getPresShellId(out uint32_t aPresShellId);
+  uint32_t getPresShellId();
 
   /**
    * Following modifiers are for sent*Event() except sendNative*Event().
    * NOTE: MODIFIER_ALT, MODIFIER_CONTROL, MODIFIER_SHIFT and MODIFIER_META
    *       are must be same values as nsIDOMNSEvent::*_MASK for backward
    *       compatibility.
    */
   const long MODIFIER_ALT        = 0x0001;