Massachusetts is an at-will employment state. This means that an employee can be terminated with or without a reason unless they have an employment contract that says otherwise.
There are some important exceptions. You cannot be fired for an unlawful reason. Unlawful reasons include:
Discrimination based on your membership in a protected class. Protected classes include race, gender, national original, disability, sexual orientation, and more.
Retaliation for protected activity. Protected activity includes taking federal or state family medical leave, filing for workers compensation, or objecting to or complaining about wage and hour violations or discrimination or sexual harassment. There are other protections specific to certain kinds of workplaces. For example, there is a statute that protects health care workers from retaliation for reporting violations, and another that protects public employees from complaining about or refusing to engage in unlawful activities.
Minority shareholder freeze-outs: If you are both an employee and a minority owner of a closely held company, firing you without a good reason could be unlawful as a breach of fiduciary duty.