The critical issue isn't where the gun is, it's where your mother resides. If she's a Maine resident, then the transfer will have to go through a Massachusetts FFL. If she's a Massachusetts resident, then she either needs to have an LTC (A or B, as appropriate for the gun) or do the transfer through a Massachusetts licensed dealer. You, of course also need the appropriate LTC. [OTOH, if you and your mother were once Maine residents, for all I know she might have given the gun to you then, in which case you could simply bring your gun into the state and register it with an FA-10 without any new transfer taking place.]
Ken