A framing corner joint includes first and second framing rails of fiberglass-reinforced resin construction. The first and second framing rails have hollow mitered ends. A pair of thermoplastic plugs are received in the mitered ends of the respective framing rails. Each of the plugs includes a body inserted into the hollow interior of an associated framing rail and a flat plug flange at an angle of 45° to the body. The plug flanges extend outwardly from the peripheries of the bodies between the rail ends and have flat end faces that are bonded to each other. The bodies of the plugs preferably are hollow, and preferably are received by interference press-fit within the ends of the rails. The plug flanges of the plugs preferably are welded to each other.