For any cross browser problem, the first thing to check is whether you have validation errors in the core being output.
Nowadays browsers do a fairly good job of rendering correctly structured code consistently. So differences are almost always because of coding errors.
Applying this approach to your product page shows that you have nested a second form (wishpot) inside the main form and worse, the code isn't xhtml compatible so the browsers are seeing incomplete input fields. Some browsers are managing to make sense of this (probably by ignoring it completely, others not - hence the inconsistency that you're seeing).
So this is nothing to do with stock by attributes and instead is related to other changes that you have made to your product page.
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