> That’s weird because you’ve been able to use their embedded web maps for a few years now just fine with Firefox. Wonder what gotcha they ran into implementing the full thing that needed a browser check?

Likely nothing. "Unsupported" messages like that are usually not written based on what the website/webapp can run on, but rather what they have/not have testing for. So if they're only testing it on Windows/macOS/Chrome/Safari, even if their developers probably confirmed it works in Firefox/Linux, they'll add that message/block as their QA doesn't include Firefox or Linux.