Incorrect Name On Order Notification

This should be an easy problem to fix.

At the moment if a customer uses a different shipping address from their billing address the name that appears on the confirmation email i.e. "Dear john, your order has been accepted" seems to be the name form the shipping address.

Looking at the order notification template this is the code

{__("dear")} {$order_info.firstname}

Does anyone know the correct value to replace here to pull in the name from the billing address instead?

{__("dear")} {$order_info.b_firstname}