This code fixes the issue in Firefox where the onreadystatechange event is not called for synchronous XHR requests. It is based on the XHConn script from http://xkr.us/code/javascript/XHConn/, patched with the fix documented at http://lukav.com/wordpress/2007/04/12/firefox-firebug-and-synchronos-calls-problem/

   1  
   2  /** XHConn - Simple XMLHTTP Interface - bfults@gmail.com - 2005-04-08        **
   3   ** Code licensed under Creative Commons Attribution-ShareAlike License      **
   4   ** http://creativecommons.org/licenses/by-sa/2.0/                           **/
   5  /**
   6   * Modified slightly from original to support synchoronous transactions
   7   * CDB 2007-05-16, 2007-06-05
   8   */
   9  
  10  function XHConn()
  11  {
  12  	var xmlhttp, bComplete = false;
  13  	try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); }
  14  	catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); }
  15  	catch (e) { try { xmlhttp = new XMLHttpRequest(); }
  16  	catch (e) { xmlhttp = false; }}}
  17  	if (!xmlhttp) return null;
  18  	this.connect = function(sURL, sMethod, sVars, fnDone, bAsynch)
  19  	{
  20  		if (!xmlhttp) return false;
  21  		bComplete = false;
  22  		sMethod = sMethod.toUpperCase();
  23  
  24  		if (bAsynch == null) bAsynch = true; //treat asynch as an optional argument
  25  
  26  		try {
  27  			if (sMethod == "GET")
  28  			{
  29  				xmlhttp.open(sMethod, sURL+"?"+sVars, (bAsynch == true));
  30  				sVars = "";
  31  			}
  32  			else
  33  			{
  34  				xmlhttp.open(sMethod, sURL, (bAsynch == true));
  35  				xmlhttp.setRequestHeader("Method", "POST "+sURL+" HTTP/1.1");
  36  				xmlhttp.setRequestHeader("Content-Type",
  37  					"application/x-www-form-urlencoded");
  38  			}
  39  
  40  			xmlhttp.onreadystatechange = function(){
  41  				if (xmlhttp.readyState == 4 && !bComplete)
  42  				{
  43  					bComplete = true;
  44  					fnDone(xmlhttp);
  45  				}
  46  			};
  47  			xmlhttp.send(sVars);
  48  
  49  			/**
  50  			 * Firefox <= 2.0.0 doesn't fire onreadystatechange for synchronous requests.
  51  			 * See http://lukav.com/wordpress/2007/04/12/firefox-firebug-and-synchronos-calls-problem/
  52  			 */
  53  			var isGecko = (document.addEventListener) ? true : false;
  54  			try {
  55  				if (!bAsynch && isGecko && xmlhttp.onreadystatechange == null) {
  56  					bComplete = true;
  57  					fnDone(xmlhttp);
  58  					return true;
  59  				}
  60  			}
  61  			catch (e) { return false; }
  62  			/**
  63  			 * End synchronous request patch
  64  			 */
  65  		}
  66  		catch(z) { return false; }
  67  		return true;
  68  	};
  69  	return this;
  70  }

// Snippet showing how to use synchronous AJAX

   1  
   2  function getFile(url) {
   3    if (window.XMLHttpRequest) {              
   4      AJAX=new XMLHttpRequest();              
   5    } else {                                  
   6      AJAX=new ActiveXObject("Microsoft.XMLHTTP");
   7    }
   8    if (AJAX) {
   9       AJAX.open("GET", url, false);                             
  10       AJAX.send(null);
  11       return AJAX.responseText;                                         
  12    } else {
  13       return false;
  14    }                                             
  15  }
  16  
  17  var fileFromServer = getFile('http://somedomain.com/somefile.txt');