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.