Everything worked perfectly as it always does.
- 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.