Although “fire and brimstone” do rain down on Sodom and Gemorrah in Genesis, there are other places where only fire rains down from the sky. (In Leviticus, fire from the sky kills two priests because they made the wrong kinds of holy offerings; in Elijah, the prophet successfully draws down fire from heaven to ignite his to-be-burned offerings.)
I’m guessing that the “fire from the sky” in these sections was an early interpretation of lightning. Before Ben Franklin flew his kite in a storm, so the story goes, everybody thought that lightning wasn’t an electrical phenomenon, and was in fact made of fire (as evidenced by how trees or wooden houses struck by lightning would burn).