In Firefox when I go to the product detail page and enter a different quantity, one that has been entered before where there's an autopopulate drop down that appears, and then click add to cart it doesn't add it to the cart.

 

I tested this on Smith's demo site and couldn't replicate. The only difference being that yours doesn't show the auto populate option. I can see that when you change the quantity something is being posted back or updated so I assume that's where things are going wrong.

 

Why does your store not use the browser auto populate feature? Is this where the problem is coming in?