Wp e-commerce shopping cart is empty error only in firefox, how to fix it?

In the website I use wordpress and wp e-commerce plugin. And I have the problem going through the checkout page: when I want to pay with paypal it shows Error detected: your shopping cart is empty. It’s interesting cause I don’t have this problem in Safari and Chrome.

I tried to clean all cache and proceed the gates and I get the same error, also asked some people to check through their computers and in Firefox they face the same problem while in Safari or Chrome there isn´t such a problem.

The page I’m talking is: http://www.mh2.com.pt

Does anyone know why it can happen like this and how is it possible to fix?