Vincenzo Bruscella died on Wednesday after suffering burns to 85 percent of his body, Ansa reported.
His 43-year-old co-owner, Michele Bruscella, died on Sunday.
Seven people were killed on the spot and several others injured in the explosion in the southern port city of Bari last Friday.
Another person who was seriously injured in the blast died on Saturday.
Italian President Sergio Mattarella said everything would be done to ensure that the cause of the tragedy would be discovered "as soon as possible".
Two Canadair water-bomber planes were deployed to stop the fire triggered by the blasts reaching a nearby forest.
The factory was totally destroyed.
Firework factory explosions are fairly common in Italy, especially during the summer.
In May, three people died and three others were injured in an explosion at a fireworks factory near Naples.
In June 2013, one person was killed in a blast at a factory in Pescara.