That's excellent advice that many have learned the hard way... Honestly even as a non-manager it's a very bad idea. When I was a junior one of my colleagues was called to the manager's office. I assumed it would be for some mundane project scheduling or whatever, so I jokingly said "you're getting fired" as he was going there. And he was. This is one of these memories that come back to haunt you late at night when yo…
Off topic. Do firings come out of the blue though? I was a VP for a while (not anymore) - but when the person I had to fire was about to get fired, they KNEW it as soon as the door opened. Firing someone is the worst experience ever - the only reason I had to fire someone was because WE screwed up in the interview process.
If someone simply isn't working well enough for their role, there is usually some kind of capability process after which their employment might be terminated but we wouldn't usually call that being fired and if that is made clear in the job description and in regular feedback, I don't think most people would consider that unreasonable.