Cleaning up https and SSL errors on site with shopping cart

if you visit this site using your CHROME browser and you add one of the products to the shopping cart, then click the “Proceed to Checkout” button on the Cart page, you’ll find yourself on the first Checkout page.

At the top of this page there’s an X through https indicating an unsecure checkout. How can I find out what links are causing that to trigger? If I look at the source code, is it those links that start with “http://”? Is there a good tool to tell? I have firebug installed but not don’t see any tab of it that lists the specific errors.

please advise. Thanks in advance!