Never been a fan of power trip humor. If you actually have the authority to fire someone, you should never ever say "you're fired" to them, even as a joke, unless you are actually firing them, and that should be done in private.
Well this guy was in a league of his own. He openly made jokes about a fellow employees lazy eye in public and once made a semi-racist off color joke to begin another all-hands. I'm pretty sure he was left off of the investor quarterly calls for being such a loose cannon. Amazingly enough he left on his own and didn't have any openly public HR complaints or lawsuits against him.
Sometimes my former boss would tell me I was fired if I made a bad joke when we were out drinking . . . but in general, I agree that it should not be done if there is any ambiguity at all in the situation.
And even in private there are Mich better ways of saying it. "You're fired" is unnecessarily hostile.