BOSTON — The highest court in Massachusetts has ruled that prosecutors must reduce a first-degree murder conviction or grant a new trial to a woman who set a fire that killed a mother of four.The Supreme Judicial Court on Tuesday upheld Chiteara (shuh-TEE'-ruh) Thomas' conviction for arson, but said her first-degree murder conviction cannot stand.Prosecutors said Thomas used a cigarette lighter to start the 2006 fire after a dispute with a first-floor tenant.