I’ve never seen an alert either, but I’ve seen plenty of sites and pages that only work in Chrome. Just yesterday I was trying to pay a credit card bill and the page kept breaking in Firefox. Out of desperation I tried it in Chrome and it worked.

Do Slack huddles still require Chrome? That was the most recent one I recall bumping into

I've almost exclusively seen them on "luxury" apartment websites, where they're completely ignorable . oh, and tidepool, a diabetes data visualization website, where it is actually required.

A bank I use rewrote all their sites, bigger fonts, much slower and blocks firefox, and it's not just user-agent.

There are a few government website where I live that block Firefox, although those are just checking the user-agent so easy to work around.

And yes I complain all the time about it.