Ok - trying to figure this out. I did some searching in the DTK to see what I could find. The error is (and I'm sure you know this!) ENTRY_FIRST_NAME_ERROR, and I looked at these files (thinking one of these might be the culprit):

includes/modules/pages/checkout_shipping_address/jscript_main.php
includes/modules/payment/authorizenet_aim.php
includes/modules/checkout_new_address.php
includes/form_check.jp.php

I compared these files from my live site with the original full file set for 1.3.6, and the files are all identical, except the authorizenet_aim.php where I had to add in some code to deal with godaddy CURL stuff.

I also searched for x_ship_to_first_name, and it looks as if it's only showing up in the authorizenet payment module files.

Anything anyone can suggest to try?