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).
Yes, there are certainly lots of references to fire in the Bible. As mentioned in the Mailbag item, fire is viewed as a purifier. I chose not to go in that direction in any depth, because the question was focused on brimstone, not on fire, and not on Sodom. Certainly the imagery of fire raining from heaven would be an interesting study.
On whether “fire from heaven” is lightning: I very carefully do my best NOT to interpret Biblical verses in terms of modern scientific explanations. Many learned and noteworthy people do, I’m not dissing them, I just think it’s fruitless. I stick to the literary text, which talks about fire and brimstone falling from heaven to destroy Sodom and Gomorrah. You can argue that it was a volcano, you can argue that it was a nuclear blast, you can argue that it was an attack by X-files-type aliens. All those theories – and more – have been offered.
I limit my discussions to the poetry and literary text.