the validated entries will only be set if the form fails validation.
In the validation server behaviors, set the failed redirect to go back to the checkout page and pass "failed=true" as a querystring variable:
and use a ternary expresion to use the validated entries based on the query srting:
value="<?php echo((isset($_GET['failed']))?(ValidatedField("confirm","firstname")):(isset($_SESSION['eCartCheckoutForm_firstname'])?$_SESSION['eCartCheckoutForm_firstname']:"")); ?>"