You cannot be fired for participating in a protected strike or picketing, depending on the purposes and means of the strike action.
Under federal law, you cannot be fired for participating in a protected strike or picketing against your employer. There are limitations and qualifications on the exercise of that right. Most strikes are protected, but certain kinds of strikes are not protected, depending on the object or purpose of the strike, on its timing, or on the conduct of the strikers. You can be lawfully fired for participating in an unprotected strike.
When a protected strike ends, you are entitled to return to work. If the reason for the strike was, in whole or in part, to protest one or more unfair labor practices, strikers must be immediately reinstated. If the strike was over economic issues, you are likewise entitled to immediate reinstatement except that if your employer hired permanent replacements, returning strikers are placed on a preferential hiring list. Your right to reinstatement may be lost if you have engaged in violence or other serious misconduct in connection with your strike or picketing activities.