N.J. Stat. Ann. § 40A:14-29

In any municipality except in cities of the first class a promotion of any member or officer of the paid or part-paid fire department or force to a superior position shall be made from the membership of such department or force. No person shall be eligible for promotion to be a superior officer unless he shall have previously served as a permanent paid fireman for at least 3 years in such department or force.