ADD to Card in not working

[dawebservices]

i install the VirtueMart in my joomla 1.5 but the add to card is not working.
add to card button in the site is working but in the main page shows me "Choose  a product variant first"
how can i solve this problem.
this problem in Firefox happens.
this is link to my website:
http://www.cyprusbasket.com/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=17&virtuemart_category_id=10&Itemid=2

[jenkinhill]

Add to cart works for me on that page, Firefox 13.0.1