It automatically appears upon a new Layer

Swap the Blend Mode for the Firework Layer to 'Screen' (see below)

Areas of black in the firework image will be invisible whilst bright areas will become visible over the black night sky on the Background image.

If you need to alter the size of the firework,. use the Edit > Transform menu or 'Quick Key' Ctrl+T for 'Free Transform.

Refinements can be made by adding a Layer Mask to the Firework Layer (see below)

Refer to Know-How contents should you need information about using a Layer Mask (look under 'Layers') .
Choose 'Layer Mask' in Table 1 or 2, depending upon which version of Photoshop you're using.