They are lighting us up like a firestorm.

The keyword here is "firestorm." During the second world war, RAF Bomber Command learned how to create firestorms, using conventional high explosive bombs along with incendiaries to cause a conflagration which would continue long after the bombers were gone. The fires cause a huge updraft, and that draws air in from the area surrounding the bombed city, creating a tornado of fire. In the context of a firestorm, "lighting us up" refers to the volume of fire from the enemies weapons, and the officer is employing hyperbole. exaggeration, to suggest that the volume of fire from the enemies' weapons is creating a firestorm. This is the number one definition from Merriam-Webster for firestorm:

1 : a very intense and destructive fire usually accompanied by high winds

The Rooses' house and gardens were obliterated in the 1991 Oakland firestorm. —Craig Summers Black;

especially: one that is started by attack with nuclear or incendiary weapons and that creates a powerful updraft which causes very strong inrushing winds to develop in the surrounding area;

His goal was to create firestorms like the ones that had consumed Hamburg and Dresden, conflagrations so vast and intense that nothing could survive them … —David M. Kennedy

The officer is comparing the amount of fire (in the sense of the enemy shooting at them) they are taking to such a firestorm.

Lighting us up = idiom meaning "Giving us everything they've got."