I was slandered by a customer on social media. I privately messaged them and politely defended myself, and was fired from work for doing it. Is this wrongful termination?

It most likely is and while reinstatement is not an option, you are entitled to pay in lieu of notice. Most people's notice entitlements are commensurate with their years of service, age and position unless they signed enforceable terms that say otherwise. If you are looking for assistance in the matter against your former employer please contact us at the number below.