火狐下js远程访问页面内容，不行，寻找原因，IE可行。

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>js实现远程访问页面内容</title></head>
<body>
  <div id="DynamicContent_id">
        .显示在这里
    </div>
     <script language="javascript" type="text/javascript">
       
       function getOs(fragment_url, element_id)
       {
         var varxml="";
         if(navigator.userAgent.indexOf("MSIE")>0)
         {
           var element = document.getElementById(element_id);
           varxml= new ActiveXObject("Msxml2.XMLHTTP");
           varxml.open("POST", fragment_url);
           varxml.onreadystatechange = function()
           {
             if(varxml.readyState == 4 && varxml.status == 200)
             {
              var txtconent = varxml.responseText;
              alert(txtconent);
             }
              
           }
           varxml.send(null);
         }
         if(navigator.userAgent.indexOf("Firefox")>0)
         {
          var element = document.getElementById(element_id);
           varxml=new XMLHttpRequest();
           varxml.open("POST", fragment_url);
           varxml.onreadystatechange = function()
           {
             alert(varxml.status);
             alert(varxml.responseText);
             if(varxml.readyState == 4 && varxml.status == 200)
             {
              var txtconent = varxml.responseText;
              element.innerHTML = txtconent;
             }
              
           }
           varxml.send(null);
         }
       }
          getOs("http://house.china.com.cn/ajax.aspx?oper=getOtherUrl&url=http%3A%2F%2Fu.house.china.com.cn%2Fbbs%2Fforumdisplayxmllistcq.php%3Ffid%3D519%26num%3D15", "DynamicContent_id" );
    </script>   
</body>
</html>在火狐下status的值总为零，寻求解决方法。