Ext在firefox中出现uncaught exception.....(XMLRequest.Open)的异常!

这是因为firefox安全性强，不允许跨域调用。

Firefox 要取消XMLHttpRequest的跨域限制的话，
第一是从 about:config 里设置 signed.applets.codebase_principal_support = true； （地址栏输入about:config 即可进行firefox设置）
第二就是在open的代码函数前加入类似如下的代码：
try { netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead"); } catch (e) { alert("Permission UniversalBrowserRead denied."); }


<script type="text/javascript" language="javascript">

// Error: uncaught exception: Permission denied to call method XMLHttpRequest.open

  var http_request = false;

  function makeRequest(url, parameters) {

   try {
    netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");
   } catch (e) {
    alert("Permission UniversalBrowserRead denied.");
   }

    http_request = false;
    http_request = new XMLHttpRequest();
    if (http_request.overrideMimeType) {
      http_request.overrideMimeType('text/xml');
    }
    if (!http_request) {
      alert('Cannot create XMLHTTP instance');
      return false;
    }
    http_request.onreadystatechange = alertContents;
    http_request.open('GET', url + parameters, true);
    http_request.send(null);
  }

  function alertContents() {
    if (http_request.readyState == 4) {
      if (http_request.status == 200) {

        var string = http_request.responseText;
      alert(string);

      } else {
        alert('There was a problem with the request.');
      }
    }
  }
  function updateweather() {
    makeRequest('http://www.wunderground.com/auto/rss_full/global/stations/16239.xml', '');
  }
</script>

<input type="button" name="button" value="GET XML"
  onclick="javascript:updateweather();">