XmlHTTP Request Returns 403 In FireFox When Post Used

Currently transitioning from a shared host to a dedicated server. The
same code that works on the old server is not working on the dedicated
server. It is a simple AJAX request like:

<code>
function createXMLHttpRequest() {
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}


function doRequestUsingPOST() {
createXMLHttpRequest();
var queryString = "response.php?";
queryString = queryString + createQueryString() ;
xmlHttp.onreadystatechange = handleStateChange;
xmlHttp.open("POST", queryString, true);
xmlHttp.send(null);
}
</code>

With the code as is above, the requests works fine in IE. With Firefox
it throws a 403 on the page. A call to the response.php page with
parameters runs correctly outside of AJAX. Changing all POST requests
to GET resolves the issue, but I would prefer not to have to change
ALL POST requests to GET requests.

Does anyone know of a setting on the new server that can cause FireFox
(1.5.x and 2.0.x) to return a 403 with an AJAX post call?