So....when you completed your W-4, what did you tell them to take out?
An employer takes out the taxes that YOU tell them to take. If not enough was taken out, it's no one's fault but yours.
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.