Last versions of XenForo require minimal PHP version 5.3.3. It means that it's already possible to use late static binding and it's possible to use static method by 'static'. That's why it's time to replace self::method() to static::method(). Also it's reasonable to replace self to static in description of callbacks, for example, in XenForo_Template_Helper_Core::$helperCallbacks, etc. It will make easier extending of static methods in addon development.