The fire step, is just like is says. In order to fire their weapons, the soldiers had to take a step up to fire. The step that they had to stand on became known then as the "fire step".
This diagram shows a fire step that the soldiers had to stand on to fire their weapons in order to fire above the sandbagged parapet.
Towards the middle of the picture you can see the soldiers fire step that he would stand on in order to fire his weapon at the enemy.