I was wondering the same question not long ago. Turns out there are plenty of situations where firing someone is illegal, and most of them have to do with the reason why you fired that person. For example, it is illegal to fire someone for discriminatory reasons. This means that even if your employee’s contract specifies that you can fire them at will if you fire them for reasons related to their race, religious belief, gender, sexual preference, or a reason related to any other category protected by law, you are breaking the law.
For example, firing someone because they got pregnant is considered gender-based discrimination and is illegal. That person will be able to contact an employment and securities litigation attorney like Girard Bengali APC and take legal action. They may be entitled to compensation, and the courts may even ask that you reinstate that employee. It’s also illegal to retaliate against and fire whistleblowers.