Try posting at the Web Development / Standards Evangelism forum at MozillaZine. The helpers over there are more knowledgeable about web page development issues with Firefox. http://forums.mozillazine.org/viewforum.php?f=25 You'll need to register and login to be able to post in that forum.

In Firefox, you cannot request other websites (websites that do not use relative path, like http://www.google.com) on client side.

I got the same issue, but with Firefox 12. I use this code to call my COMET server:

           function getXmlHttpRequestObject() {
               if (window.XMLHttpRequest) {
                   //document.getElementById("_receivedMsgLabel").innerHTML += "Non-microsoft xmlHttpRequest object created.
";
                   alert("Non-microsoft xmlHttpRequest object created.");
                   return new XMLHttpRequest();
               }
               else if (window.ActiveXObject) {
                   //document.getElementById("_receivedMsgLabel").innerHTML += "Microsoft xmlHttpRequest object created.
";
                   alert("Microsoft xmlHttpRequest object created.");
                   return new ActiveXObject("Microsoft.XMLHTTP");
               }
               else {
                   alert("Status: Could not create XmlHttpRequest Object.  Consider upgrading your browser.");
                   //document.getElementById("_receivedMsgLabel").innerHTML += "Status: Could not create XmlHttpRequest Object.  Consider upgrading your browser.
";
               }
           }

           var sendReq = getXmlHttpRequestObject();
           var receiveReq = getXmlHttpRequestObject();
           var lastMessage;
           var mTimer;

           //Gets the server response:
           function getResponse() {
               document.getElementById("_receivedMsgLabel").innerHTML += "getResponse() called.
";
               if (receiveReq.readyState == 4 || receiveReq.readyState == 0) {
                   //if (receiveReq.readyState == 0) {
                   //receiveReq.open("POST", 'http://tardis:1802', true, "server", "server123");
                   //receiveReq.open("POST", 'http://localhost:1802', true, "server", "server123");
                   //receiveReq.open("POST", 'http://holit109:1802', true, "server", "server123");
                   //receiveReq.open("POST", "http://localhost:1802", true, "server", "server123");
                   receiveReq.open("POST", "http://L45723:1802", true, "server", "server123");  //must use this URL at work.
                   receiveReq.onreadystatechange = handleReceiveMessage;
                   alert("handleReceiveMessage assigned to onreadystatechange event.");
                   receiveReq.setRequestHeader("Content-Type", "text/x-json");
                   receiveReq.timeout = 0;
                   var currentDate = new Date();
                   var sendMessage = JSON.stringify({
                       SendTimestamp: currentDate,
                       Message: "Message 1",
                       Browser: navigator.appName
                   });
                   //receiveReq.send("<Request><Command>Queue</Command><User>user1</User><Message>Message 1</Message><Message>Message 2</Message><Message>Message 3</Message></Request>");
                   alert("JSON message created.  About to send...");
                   receiveReq.send(sendMessage);
                   alert("Message sent.");

               }
           }

           //function for handling the return message from Comet
           function handleReceiveMessage() {
               if (receiveReq.readyState == 4) {
                   document.getElementById("_receivedMsgLabel").innerHTML += "Message received!
";
                   var status = receiveReq.status;
                   //document.getElementById("_receivedMsgLabel").innerHTML += "Status received!
";
                   var txt = receiveReq.responseText;
                   var receivedMsg = JSON.parse(txt);
                   document.getElementById("_receivedMsgLabel").innerHTML += receivedMsg.Message + "
";
                   //var receivedTime = new Date();
                   //alert("Got current date.");
                   //alert(receivedTime);
                   //var receivedTime_ms = receivedTime.getTime();
                   //alert("Got time message received in ms.");
                   //alert(receiveReq.SendTimestamp);
                   //var sentTime_ms = getDateFromFormat(receiveReq.SendTimestamp, "dd/MM/yyyy HH:mm:ss");
                   //var sentTime = new Date(receiveReq.SendTimestamp);
                   //alert("Got time message sent in ms.");
                   //var sentTime_ms = sentTime.getTime();
                   //var difference_ms = receivedTime_ms - sentTime_ms;
                   //document.getElementById("_receivedMsgLabel").innerHTML += "Comet took " + difference_ms + " ms.
";

                   mTimer = setTimeout("getResponse();", 0);
               }

               getResponse();
           }

The handleReceiveMessage() event handler is called, but there is no data returned in the responseText property of the xmlHttpRequest object.

Why?

This works fine in IE9.