What are some of the potentially “illegal” reasons upon which an employer can not fire an employee?

There are both federal and state laws that protect employees from discriminatory employment decisions which are based on race, national origin, sex, religion, age, and disability.  There are also statutes that prevent retaliation in connection with an employee reporting illegal activities, filing a workers’ compensation claim, attending jury duty, requiring child support deductions from their pay, and some other specific activities that the law deems “protected”.