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.
It's not super difficult to just make a standards compliant website. I always wonder how in this day and age people manage to create professional websites with browser specific bugs.
Just visited the site with FF, and got no such error. It's a Shopify site, and I'm sceptical.
If it's a typical Shopify SBO, it could easily be a single person - the owner - working out of their house. There is no developer, except those employed by Shopify.
The owner probably populated the store themselves; the entirety rest of their computer experience brobably consists of browsing Wiccan forums, Instagram, and Twitter. And yet, they figured out how to open an online shop and start a business doing something they're passionate about.
Educated guesses, but poking around a bit on the site & following links gives good evidence this person is a person, not a company, and doesn't employ anyone, much less programmers.
And I've never had a Shopify site pop up a message like this. I think OP hit a fluke, or a MITM, or (most likely) has a virus.
It’s time to get rid of the part of user-agent strings that identifies which browser you’re using. It should only include things like mobile/desktop, version of html supported, and JavaScript version supported.
That store is hosted on Shopify. I'm quite confident they handle Firefox just fine. The customization(s) the company made to defaults on the other hand...
I see someone wearing that "Necromancer Trench Coat" (when it becomes available) and I'm going to assume they're hiding a shotgun under it and are thinking of undertaking their own Columbine