A firefighters’ organization is offering a $5,000 reward to help solve a string of at least 15 arson fires reported in Plympton and other communities south of Boston in the past month, including Easton, Halifax, Hanson, Kingston, Middleborough, Quincy, Raynham, and Weymouth. The Professional Fire Fighters of Massachusetts, which represents more than 12,000 firefighters, is seeking information leading to the arrest and conviction of anyone responsible for setting the fires, mainly set in unoccupied buildings. Federal investigators are helping state and local fire officials look into the suspicious fires. “We’ve been lucky that no lives have been lost up until now, but the longer these fires continue, the greater likelihood that tragedy will strike,” said Edward Kelly, president of the statewide firefighters’ group. Tips can be phoned into the Arson Hotline at 800-682-9229.