Bug #372671 --> "restore default" doesn't restore the default start page. sr=bienvenu

mail/components/preferences/general.js

@@ -99,7 +99,8 @@ var gGeneralPane = {
   {
     var startPage = document.getElementById("mailnews.start_page.url");
     startPage.value = startPage.defaultValue;
-    startPage.setElementValue(document.getElementById("mailnewsStartPageUrl"));
+    if (startPage.hasUserValue)
+      startPage.reset();
   },
   
   /**