[swores]

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.

[stnvh]

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.