Why do you think that firing someone over this is the correct response? I'm sorry but that is a really stupid mindset.
You learn and train/educate your employees so that it doesn't happen again.
“Recently, I was asked if I was going to fire an employee who made a mistake that cost the company $600,000. No, I replied, I just spent $600,000 training him. Why would I want somebody to hire his experience?”
– Thomas John Watson Sr., IBM