In all states I am aware of (I am licensed only in Virginia) the definition of a firearm turns on the presence of a barrel, a projectile, and the propellant. A flare gun has all the requisite basics to be considered a firearm. Although you should run down your states' definition online easily enough, I suspect you will find it IS a firearm.
Whether YOU can carry a firearm depends on your state's laws regarding carry, conceal and carry, and whehter or not you have any disabilities that would prevent you from possessing a firearm (felony convictions, history of mental issues, DV, etc.).
Best option is for you to consult with a MA attorney directly for a read on what your state allows and what your specific situation is.
NOT LEGAL ADVICE. FOR EDUCATION AND INFORMATION ONLY. DO NOT RELY ON ANY ADVICE YOU RECEIVE FROM ME OR ANY OTHER ATTORNEY IN THIS FORUM. Legal advice comes after a complete review of the facts and relevant documents and an expressed (written) agreement of representation that forms attorney-client confidentiality. Neither of these two events can occur in this forum. Mr. Rafter is licensed to practice in the Commonwealth of Virginia and the US Federal Courts in Virginia. His answers to any Avvo question are rooted in general legal principles--NOT your specific state laws. There is no implied or actual attorney-client relationship arising from this education exchange. You should speak with an attorney licensed in your state, to whom you have provided all the facts before you take steps that may impact your legal rights. Mr. Rafter is under no obligation to answer subsequent emails or phone calls related to this or any other matter.
A flare gun is considered a firearm. You can carry it on your person, at least on land, under he same conditions and circumstances as you could carry a revolver or semi-automatic pistol.
On a vessel, however, in addition to being a firearm, it may also be a required piece of safety equipment. Consider your standard, run-of-the-mill, 12-gauge flare gun (see link below). It is approved by the USCG (the Department of Homeland Security!) as required safety equipment, but it is also a firearm and, in many states, meets the definition of a illegal "short-barrelled shotgun." If you carry it aboard your boat on navigable waters, there is almost a certainty that you will not have a problem. Get caught carrying the same flare gun under your shirt in a convenience store, and you had better have either a permit or a good criminal defense attorney..