I need to know if it is legal for a company to lay off full time tenured employees and fill said spots with immigrant workers. This is a common practice with this company, and with that being said, I don't even know if these people are legal.

If the "tenured" employees don't have a contract then this would be perfectly legal. If the company is hiring illegals then they could face sanctions. But as you said...You don't know if they are illegal.

Do you work for a private company or a public agency? By "tenured" do you mean you are a union member?