You could just ask the French. The harder it is to fire employees, the harder it is to find a job in the first place.

At the limit, you would just reach a no hiring, no firing steady-state with a minimally functioning economy.

The only activity would be new companies hiring until they were overwhelmed by dead weight and companies going bust. (Wouldn't that be a little like firing everyone?) Strange and ponderous unintended consequences lie here!