Slack is guilty of this as well. They support Firefox obviously, but if you try to connect with a niche browser like Waterfox you get a great patronizing message about how it's for your own good and they only want to provide you the "best experience possible."

The best part is that the default Waterfox user-agent begins with an unmodified copy of the Firefox one (which is allowed), with " Waterfox/X.Y." appended to it.