532733 osCommerce One Page Checkout Popup Issue

We have installed a One Page Checkout contribution to our oCommerce store, but have one small problem with it in Firefox. After entering address and payment info, upon clicking the "Confirm Order" button a popup appears stating "There was an error updating your billing address..." But the billing address is already there. It then refreshes the checkout page and removes the billing address. The checkout page is working perfectly in IE but we experience this issue with Firefox only. Also, we created a function where the user could not checkout without entering all their information - as we had some issues with this initially. We must make sure that this remains in place with any new fix for this current remaining issue. Someone who can start immediately will be preferrable. Thank you.