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.
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.
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.