If you try to access Apple Maps on an unsupported browser, you get a hard "Your current browser isn't supported" block, presumably implemented with UA detection.
https://beta.maps.apple.com/asd as an example.
Their devs are probably trying to make sure it runs in Firefox but they haven't added it to the official QA process, just like they haven't added languages other than English either. And they don't want Firefox users complaining about bugs that might be there yet.
If this were a full release then sure I'd complain too. But this is just a beta. Maybe releasing betas before they have full compatibility and internationalization is good, rather than "miserable"?
Blocking untested combinations seems contrary to that goal.
Or I guess it's another example of how the word "Beta" has become marketing speak for "New!", with a side of "No Support!"
There are phases of internal and more limited testing that need to precede the broader beta phase.
If you haven't finished those phases yet on other browsers, there's no point in jumping ahead to a public beta for them.
You wouldn’t have given this a second of extra thought if it wasn’t done by a company you’ve obviously long-since decided that you hate for cultural reasons.