firefox 3 call Components.classes 错误

firefox 3 call Components.classes

在firefox3下Components.classes 是不允许直接调用的，需要加上如下那句粗体的语句才可以
<script>
netscape.security.PrivilegeManager.enablePrivilege('UniversalXPConnect');
var prefs = Components.classes['@mozilla.org/preferences-service;1'].getService(Components.interfaces.nsIPrefBranch);
prefs.setBoolPref("dom.allow_scripts_to_close_windows", true);
</script>