Preheat oven to 375°F. Bring a large pot of water to a boil over high. Stir in rice; reduce heat to medium, and cook, uncovered, until rice is tender, 18 to 20 minutes. Drain.
While rice cooks, toss together bread cubes and 2 Tbs. of the melted butter on a rimmed baking sheet; spread in an even layer. Bake until golden and crisp, 12 to 14 minutes. Let cool completely.
Meanwhile, heat oil in a large nonstick skillet over medium-high. Add celery, carrots, onion, and bell pepper; cook, stirring occasionally, until vegetables start to soften, about 5 minutes. Add pumpkin; cook, stirring often, until vegetables are tender, about 15 minutes.
Stir broth, scallions, lemon juice, salt, pepper, bread cubes, cooked rice, and remaining 4 Tbs. butter into pumpkin mixture. Cook, stirring occasionally, until heated through, about 5 minutes. Serve sprinkled with cilantro.