Yes, you can be fired for not working overtime. Mandatory overtime is legal in all 50 states.
There are only two states that limit the number of hours an employee can be required to work in a week/pay period. NJ is neither of them.
Ziggie has correctly provided the legal recourse.
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.