Employers in states that follow the "at-will" rule of employment can generally fire any employee at any time for any reason, or even for no reason at all. Even so, they cannot fire employees solely because of the exercise of legal rights such as filing workers' compensation claims. An employee who can prove that such filing was the reason for his or her firing may have a case for unlawful retaliation.
I suggest contacting a Massachusetts employment law attorney to review the facts of your situation. Most employment claims are subject to strict time limits that may be quite short, so it is extremely important to consult with a lawyer promptly. Check lawyers on Avvo or contact your state bar association for help with a referral.
This answer is made available by an attorney licensed to practice in the state of Oregon. It is provided for informational purposes only, is not legal advice, and does not create any attorney/client relationship. Actual legal advice can only be provided by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.