Problem with AJAX in FF:synchronous request doesn't work

[=vd=]

I have folowing AJAX connector:

function ajaxConnect(url, postData, callBackFunction, mode) {
	var xmlObj = new Object();	
	if (window.XMLHttpRequest) {
		xmlObj = new XMLHttpRequest();
	} else if (window.ActiveXObject) {
		xmlObj = new ActiveXObject("Microsoft.XMLHTTP");
	} else {
		alert("Error initializing XMLHttpRequest");
		return;
	}
	xmlObj.onreadystatechange = function(){
		if(xmlObj.readyState == 4){
			if (xmlObj.status == 200) {				
				var elArr = xmlObj.responseXML.getElementsByTagName('root')[0].childNodes;
				if (callBackFunction != "") {					
					callBackFunction(elArr);
				} else {
					return true;
				}
			} else {
				alert("There was a problem retrieving the XML data");
				return false;
			}
     	} 
	}

	xmlObj.open("POST",url,mode);	
	xmlObj.setRequestHeader("Content-type","application/x-www-form-urlencoded");	
	if (xmlObj.overrideMimeType) {
		xmlObj.setRequestHeader("Connection","close");		
	}
	xmlObj.send(postData);
}

It called from:

	var postData = "wordlist="+encodeURIComponent(strWordList);
	var url = "/admin/spell/spell_checking.php?Lang="+Lang;
	var callBackFunction = spellResultHandle;
	ajaxConnect(url, postData, callBackFunction,false);

Where spellResultHandle - some function which handle result of the request and return it. I need to use synchronous mode because of result of spellResultHandle used below in code.

SO THE PROBLEM IS:
This code works in IE, Opera, but doesn't work in FF(1.5.0.6).
If I change mode to asynchronous - works, but it doesn't approach to programm logic.

Any suggestions would be welcome.
TIA