Potential postdoc advisor threatening to fire other postdocs Currently I am searching for a regular postdoc position. I visited a potential advisor some time ago. I had a good impression of the advisor except for one thing: he introduced one of his current postdoc to me and half-jokingly threatened to fire the current postdoc (said in my face and to the face of that postdoc) if the said postdoc couldn't improve his productivity.

Later the said advisor encountered funding problem and cannot hire me. Recently I asked him about the current status of his funding, he replied he was considering firing that postdoc so that he would have one open position.

I understand that, compared to PhD, postdoc positions emphasize performance much more. But telling a potential postdoc candidate about firing a current postdoc? I'm a bit scared. It sounds like a warning to me as well: if I get hired as a postdoc to replace the fired postdoc, I will get the same treatment. If the current postdoc survives and I get hired as well, this could create tension between the current postdoc and me. Is this something an advisor should tell his potential postdoc candidate? What does this tell about the advisor's personality?
Can a PI fire a postdoc? 
Surely they can, and sometimes they really have to do so in order to save research money and do the best for the group.
Can PI signal to the postdoc that their performance is under review?
Yes. This is an honest thing to do. It gives postdoc a chance to improve their performance and fix things, before guns will fire.
Is it a good idea to tell this as a joke? 
Absolutely not. The job contract is a serious personal matter. The discussion should be private and professional, not public and semi-serious.
What can we tell about PI's personality?
He cares about the performance of the group and he tries to manage it. He doesn't know/feel what is a professional way of doing it. Perhaps, he struggles to maintain a distance and tends to become a bit too friendly with people in his group. As a result, it is hard for him to communicate difficult decisions and unpleasant messages and he tries to disguise them as a joke.