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.
I just visited with FF and got the error. Looking at the console, Firefox complains about some cookies misusing specific site attributes and actively rejects some cookies from that website entirely. That might be the source of the issue with the site's "developer."
Makes you wonder had the owner even managed to get enough code together to check the UA for Fx detection that launches a dialog window as I doubt Shopify has any built-in UA detection tools like this.
I remember thinking "hah, business majors, don't they know everything is gonna be ruled by tech?" And then it turns out the tech nerds just work for the business majors still.