diff --git a/xpfe/appshell/nsIXULWindow.idl b/xpfe/appshell/nsIXULWindow.idl
index 9dc0d7c62863..3147aa111679 100644
--- a/xpfe/appshell/nsIXULWindow.idl
+++ b/xpfe/appshell/nsIXULWindow.idl
@@ -26,7 +26,7 @@ interface mozIDOMWindowProxy;
 
 native LiveResizeListenerArray(nsTArray<RefPtr<mozilla::LiveResizeListener>>);
 
-[scriptable, uuid(d6d7a014-e28d-4c9d-8727-1cf6d870619b)]
+[builtinclass, scriptable, uuid(d6d7a014-e28d-4c9d-8727-1cf6d870619b)]
 interface nsIXULWindow : nsISupports
 {
   /**
@@ -70,8 +70,8 @@ interface nsIXULWindow : nsISupports
    * Returns the difference between the inner window size (client size) and the
    * outer window size, in CSS pixels.
    */
-  [noscript,notxpcom] uint32_t getOuterToInnerHeightDifferenceInCSSPixels();
-  [noscript,notxpcom] uint32_t getOuterToInnerWidthDifferenceInCSSPixels();
+  [infallible] readonly attribute unsigned long outerToInnerHeightDifferenceInCSSPixels;
+  [infallible] readonly attribute unsigned long outerToInnerWidthDifferenceInCSSPixels;
 
   /**
    * Tell this window that it has lost a child XUL window
diff --git a/xpfe/appshell/nsXULWindow.cpp b/xpfe/appshell/nsXULWindow.cpp
index c4ec0ca22237..f7b45e6d2ddd 100644
--- a/xpfe/appshell/nsXULWindow.cpp
+++ b/xpfe/appshell/nsXULWindow.cpp
@@ -361,16 +361,18 @@ GetOuterToInnerSizeDifferenceInCSSPixels(nsIWidget* aWindow)
   return RoundedToInt(devPixelSize / aWindow->GetDefaultScale());
 }
 
-uint32_t
-nsXULWindow::GetOuterToInnerHeightDifferenceInCSSPixels()
+NS_IMETHODIMP
+nsXULWindow::GetOuterToInnerHeightDifferenceInCSSPixels(uint32_t* aResult)
 {
-  return GetOuterToInnerSizeDifferenceInCSSPixels(mWindow).height;
+  *aResult = GetOuterToInnerSizeDifferenceInCSSPixels(mWindow).height;
+  return NS_OK;
 }
 
-uint32_t
-nsXULWindow::GetOuterToInnerWidthDifferenceInCSSPixels()
+NS_IMETHODIMP
+nsXULWindow::GetOuterToInnerWidthDifferenceInCSSPixels(uint32_t* aResult)
 {
-  return GetOuterToInnerSizeDifferenceInCSSPixels(mWindow).width;
+  *aResult = GetOuterToInnerSizeDifferenceInCSSPixels(mWindow).width;
+  return NS_OK;
 }
 
 nsTArray<RefPtr<mozilla::LiveResizeListener>>