How to show only first name in magento welcome message?

Comment the welcome message code in "template/page/html/header.phtml" and Use the below code to show first name only.

$customer = Mage::getSingleton('customer/session')->getCustomer()->getData();
if(Mage::getSingleton('customer/session')->isLoggedIn()){
   echo $this->__('Welcome, %s!', $customer['firstname']);
}else{
   echo $this->__('Welcome Guest');
}