The fire from heaven shown at this point in the millennial reign of Christ is caused when the Devil is set free from the pit after the thousand years. He gathers the people of the world through lies and deceit, tempting them with all sorts of false promises and they mount an attack on the followers of Christ and more importantly the beloved city of Jerusalem. Because of this act of treason God sends fire from heaven down to destroy them and also the Devil. Man has now no excuses when he stands before God in the final Judgement as to his actions throughout history. Even with Christ reigning man can still be deceived and turn back to his sinful ways.
This fire from heaven marks the beginning of the end for the earth and mankind in his present state.