Thanks for you comments. I guess I was getting a little off-topic, as I
am making a Firefox extension, and would like to enable that extension
to use AJAX to retrieve data from other pages on that website (which
isn't mine).

All the best.

Daz.

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

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

Below is an example that can be run locally (save it to your harddisk and open the page with Mozilla/Firefox). It will fetch some RSS XML data from wunderground.com and alert it.

Always remember: XMLHttpRequest needs UniversalBrowserRead!

If the page with the XMLHttpRequest is on a http:// URI (on a webserver), it is not possible to fetch data from another domain!!! This is a security measure of Mozilla/Firefox.
Still, it comes in handy, if you can read remote data from a local application. E.g. you've got a local XUL application that needs to get data from other servers

If you're doing some extension to the Mozilla/Firefox browser, you don't need to care about permissions. Such an extension is installed into the core of the browser anyway, hence it has all privileges.
 

cross-domain-xmlhttprequest.html

<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.megasolutions.net/16239.xml', '');
  }
</script>

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

uncaught exception: Permission denied to call method XMLHttpRequest.open