You have not provided enough facts to really answer your question. However you should know that if you don't have a specific employment contract or collective bargaining agreement (union), you can be fired for any reason including no reason at all. Essentially that is what "at will" employment means and New York State has the most strict interpretation of this employment doctrine. Whether you fit into one of the limitations of this doctrine (age, race, sex, disability discrimination) will depend on the facts of your situation.
I would recommend that you contact an experienced employment attorney in your area. Many, including my firm, offer a free initial consultation so you have nothing to lose.
No attorney client relationship has been created by this answer.
Your answer will depend on why he could not abide you earning more money than he was. If it was because of your race, gender or some other protected class then you have a strong claim. If it was just a blanket belief that employees should not earn more than management then I am not certain there is very much you can do to address this. Contact an employment attorney in your area for a free consultation.
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