kan ikke slette cookie i firefox

Hej har et underligt problem.

mit logud script fejler i firefox..
har testet i chrome og safari og der virker det fint..

 class user {
 
   private $login_username = null;
   private $login_password = null;
   private $singedIn = false;
   private $useCookie = true;
   private $cookie_name = "";
   private $cookie_time = '2592000'; //30 days in sec
   private $browser = null;
   public $data = null;
 
   public function __construct() {
     $this->browser = new browser(); 
     $this->data["user_id"] = 0; // Anonymous
     $this->cookie_name = "auto_signin";
     $this->checkUserState();
   }
 
   private function readCookie() {
     if (isset($_COOKIE[$this->cookie_name])) {
       $exp = "";
       $username = "";
       $password = "";
       parse_str($_COOKIE[$this->cookie_name]);
       if (strlen($username . $password) > 0) {
         $this->login($username, $password, true);
       }
     }
   }
 
   private function writeCookie() {
     setcookie($this->cookie_name, 'username=' . $this->login_username . '&password=' . $this->login_password, time() + $this->cookie_time);
   }
 
   private function deleteCookie() {
     //FIXME: firefox fail?
     setcookie($this->cookie_name, "exp=true", time() - 3600);
     unset($_COOKIE[$this->cookie_name]);
   }
 
   public function isSingedIn() {
     return $this->singedIn;
   }
 
   private function checkUserState() {
     if (!isset($_SESSION["login"]["user_id"])) {
       $this->readCookie();
     } else {
       $this->data = $_SESSION["login"];
       $this->singedIn = true;
     }
   }
 
 public function logout() {
     $_SESSION["login"] = null;
     unset($_SESSION["login"]);
 
     $this->deleteCookie();
     $this->data = null;
     $this->singedIn = false;
   }
 
 //resten af klassen klippet væk
 }

nogen bud?