At least they seem to be working on it. Directing Firefox users to use a different browser in the mean time, temporarily, seems reasonable even if the language on that popup is a bit imprecise.

I did try adding a shirt to the cart and yeah, it added the wrong size. I’d have to switch to chrome to successfully complete an order at the moment. It’s unfortunate, but as long as they’re trying to fix it I don’t see any point in feeling outraged.