--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -909,16 +909,21 @@ XULDocument::AttributeWillChange(nsIDocu
         RemoveElementFromRefMap(aElement);
     }
 }
 
 static bool
 ShouldPersistAttribute(Element* aElement, nsIAtom* aAttribute)
 {
     if (aElement->IsXULElement(nsGkAtoms::window)) {
+        // This is not an element of the top document, its owner is
+        // not an nsXULWindow. Persist it.
+        if (aElement->OwnerDoc()->GetParentDocument()) {
+            return true;
+        }
         // The following attributes of xul:window should be handled in
         // nsXULWindow::SavePersistentAttributes instead of here.
         if (aAttribute == nsGkAtoms::screenX ||
             aAttribute == nsGkAtoms::screenY ||
             aAttribute == nsGkAtoms::width ||
             aAttribute == nsGkAtoms::height ||
             aAttribute == nsGkAtoms::sizemode) {
             return false;