Strange problem

So the problem is with my Cart(Košarica) and Login(Prijava) pages..If I put some item/product in Cart, there is a case where you get logged out/session cleared/cart emptied if you Update Cart(Ažužiraj košaricu) or if you go on checkout (Kreni na plaćanje) and you started filling the form (on email i get my cart emptied)
Theme works fine with Chrome, IE, iPhone..but it doesn’t work with mozilla browser. Plugin that I’m using is WooCommerce and the site is http://www.kelme.com.hr
Any advice is more than welcome.

Ty, Tom