log out problem

[eluser]R_Nelson[/eluser]
this is my log out code

Code:

function logout()
    {
        $this->session->sess_destroy();
        $data = array(                                
                'is_logged_in' => false
                );            
        $this->session->set_userdata($data);
        destroy_cookie();
        $this->index();
    }

Works just fine in IE but in Firefox it kicks me back in as logged in!

do i need to do something different with the destroy cookie