Can I be fired while I'm out on workers' compensation?
Unless your union contract, or individual contract of hire, requires it, an employer doesn't have to hold your job open while you can't work due to an on-the-job injury. But, the workers' compensation law does require employers to give preferential treatment in the rehiring of injured workers when they are ready to return to work.
MGL c. 152, § 75B provides protection to injured workers while they're out, or can work but have a disability. Your best bet is to consult a workers' compensation attorney or a labor law attorney.
You can also contact the Americans with Disabilities (ADA) to find out if you fall into the protected class and what your employer may be required to do. You would need to contact the ADA: (800) 514-0301 (Voice) or (820) 514-0383 (TTY) for further information.