It has been found that employees have been terminated for using profanity in the workplace through both illegal and legal means. However, it may be that the employee was fired not for using profanity but for other reasons. In fact, in some of these cases the employee had actually been fired for discussing hours, working conditions, and wages. Under the National Labor Relations Act, as long as an employee is discussing working conditions, he or she cannot be fired even if profanity is used.
See "Employers May be Unable to Fire Employees for Swearing", Whitney Fillooon, Eater, February 15, 2016