try to change your code, so it looks like this:
if($form->isvalid($formData))
{
$client= new Application_Model_DbTable_Client();
$firstname = $formData['firstname'];
$lastname = $formData['lastname'];
$email = $formData['email'];
$id=$this->getRequest()->getparam('id'); // define your id
$client->updateClient($id,$firstname,$lastname,$email); // here, change Id into proper $id
$this->_helper->redirector('index');
}