Adding the thing that complains is a one-time cost. It may work on Firefox now, but making sure that it continues working on Firefox is a continuous resource investment.
And not adding it and not testing for Firefox would cost nothing, and currently improve the user experience - because it actually just works, as a lot of things just do.
But then what if a bug in their code that only affects FF goes unnoticed due to testing, and causes significant problems for a big client, or a journalist reviewing it, or...
Personally I feel a "We don't officially support this browser, it probably works but we only test for full compatibility in <these browsers>" is a better option if you're going to go in that direction.
But I can understand why even that is a bit of a risk as if a user decides to ignore that warning and then some time later encounters a bug that, let's say, causes them to lose half a day of work, they're likely to walk away blaming the company (and maybe go round telling people they know what a shit thing it is) even if the bug wouldn't have happened had they been using one of the browsers that is fully supported and gets tested.
It seems more like the devs at MS know their code works on Firefox but have been asked by exec to push chromium(-esque) because edge is now webkit.
sidenote: this is a multi-billion $ company, no excuse to ignore any platform with their capacity, front face it looks like they can't build a good app anymore, especially if it works anyway with a simple string change in the browser - heck, web devs had to factor in ie7-8 polyfills built by the community only a few years ago. no excuse.
