FCKToolbarSpecialCombo AJAX call not working in FF

// Initialize the Month Combo
var monthCombo = function (name) {
               this.Name = name;
   var xht = getHttpRequest();
   xht.open("POST", 'plugins/monthcombo/getMenuItems.php', true);
   xht.setRequestHeader('Content-type','application/x-www-form-urlencoded');
   xht.onreadystatechange = callback_myPages;
   
   var objStr =  "";
         
   xht.send(objStr);    
}