Chnage Customer list column name in admin in magento

Chnage Customer list column name in admin 

File Path : /public_html/app/code/core/Mage/Adminhtml/Block/Customer/Grid.php


Cose :

$this->addColumn('firstname', array(


'header'    => Mage::helper('customer')->__('Name'),

'index'     => 'firstname'

));