What happens when fire comes down from heaven?
When fire falls from heaven, it comes as part of God's judgment weapons, burning up vegetation, destroying the wicked, and accompanying other plagues like hail, blood, smoke, and brimstone.
Scripture describes fire coming down from heaven as one of the weapons God has stored up for the day of His wrath. In Psalm 18 we read that smoke and fire come from His nostrils and mouth, so hot that it kindles coals, and that hailstones and coals of fire pass with His thick clouds as He rides upon the wind. This shows that fire is kept in the heavens alongside hail, ready to be released in judgment.
When this fire actually falls upon the earth, it brings burning destruction. At the sounding of the first trumpet, hail and fire mingled with blood are cast upon the earth, and as a result a third part of the trees is burnt up and all the green grass is burnt up. Ezekiel likewise describes the Lord kindling a fire in the forest that devours every green tree and every dry tree, a flame that will not be quenched, so that all flesh sees that the Lord Himself has kindled it.
Fire from heaven is also joined with pestilence, blood, hailstones, and brimstone when God pleads against His enemies, raining these things down upon them and upon the many people with them. Deuteronomy speaks of a fire kindled in God's anger that burns unto the lowest hell and consumes the earth with its increase, setting even the foundations of the mountains on fire. In the vision of the two hundred million horsemen, fire, along with smoke and brimstone, issues out of the horses' mouths, and by these three a third part of mankind is killed.
So when fire comes down from heaven, it is not an isolated event but part of God's larger arsenal of judgment, working together with hail, blood, smoke, and brimstone to burn up vegetation, strike down the ungodly, and display the Lord's fierce anger against sin.
The scriptures
The first angel sounded, and there followed hail and fire mingled with blood, and they were cast upon the earth: and the third part of trees was burnt up, and all green grass was burnt up.
There went up a smoke out of his nostrils, and fire out of his mouth devoured: coals were kindled by it. He bowed the heavens also, and came down: and darkness was under his feet.
At the brightness that was before him his thick clouds passed, hail stones and coals of fire. The LORD also thundered in the heavens, and the Highest gave his voice; hail stones and coals of fire.
Son of man, set thy face toward the south, and drop thy word toward the south, and prophesy against the forest of the south field; And say to the forest of the south, Hear the word of the LORD; Thus saith the Lord GOD; Behold, I will kindle a fire in thee, and it shall devour every green tree in thee, and every dry tree: the flaming flame shall not be quenched, and all faces from the south to the north shall be burned therein. And all flesh shall see that I the LORD have kindled it: it shall not be quenched.
And I will plead against him with pestilence and with blood; and I will rain upon him, and upon his bands, and upon the many people that are with him, an overflowing rain, and great hailstones, fire, and brimstone.
For a fire is kindled in mine anger, and shall burn unto the lowest hell, and shall consume the earth with her increase, and set on fire the foundations of the mountains.
And thus I saw the horses in the vision, and them that sat on them, having breastplates of fire, and of jacinth, and brimstone: and the heads of the horses were as the heads of lions; and out of their mouths issued fire and smoke and brimstone. By these three was the third part of men killed, by the fire, and by the smoke, and by the brimstone, which issued out of their mouths.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.