Fireworks are never easy, and this is exceptional. Nice work. It says you had the shutter open 15 seconds...is that your technique, to leave it open that long so you have multiple explosions to choose from?
I have read to use a long shutter for fireworks, besuace keep in mind that it is DARK out, and the camera is pointed towards the sky where the only light will be the fireworks. I tired a whole bunch of diffrent times. At the time, I was using a Sigma SD9 which had a maximum exposure time of 15 sec. I probabally would have used an even longer one if I could have.