Usefull focus

I'm afraid all the modes and settings that you paid all that money for are all useless when photographing fireworks and auto focus is one of them.Another way to work is to shoot all the elements separately then combine them in Photoshop. Shoot the whole scene without any fireworks then shoot the fireworks separately and drop them into your main picture in exactly the position you want them. Because the sky is black this is really easy to do.

If you leave your camera set to auto focus the lens will whirr backwards and forwards in a demented fashion trying to find something to focus on in the black sky.
Set the focus to manual and then focus on something in the far distance. Don't just wind the focus ring around to the end of its run, check it against a distant object if you can.