Strange problem

So the problem is with my Cart(Ko?arica) 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(A?u?iraj ko?aricu) or if you go on checkout (Kreni na pla?anje) 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 https://www.kelme.com.hr
Any advice is more than welcome.

Ty, Tom