Let me add some of my thoughts regarding Filming Fireworks with a DSLR. In the aspect of still photography, you are stopping down your aperture to like F16-22 because you want to compensate for your slow shutter speed because you want to capture the motion of the sparks and lights. But for video I don't think it's essential and required unless you're doing it in a creative point.
There are few variable for you to tweak in a DSLR.
Shutter speed: Having own a Mark IV, I guess the shutter speed is limited, from perhaps 1/30 - 1/4000. A good rule of thumb to is to set the shutter speed at 2x your frame rate. So if you're shooting at 1920x1080 25p. You should ideally set it to about 1/50. Of course you're not obligedto stick to it
Aperture: I feel that something around F8-11 is excellent. You can adjust it to either compensate for your shutter speed. Due to the fireworks spark's intensity, therewould be a compromise between exposing for your highlights on the fireworks sparks or the background illumination, having tested shooting on my camera before f11 is ideal! I recommend that.
ISO: Be sure to not set your ISO on AUTO mode as the camera would vary the ISO itself depending on the light level. I won't recommend that! Start off with 100, the lowest and minimal grain and noise and tweak from there.
Focus: There is no AF function for Video Mode in DSLR, make sure that you focus manually. If you cant guage the focus. set it to infinity.
Bring a tripod as well! You don't want to handheld it! A small shake is not really very desirable unless you have the intention of doing it in a creative point of view!
Just my thoughts! Hope it helped. Do share with us the footage you shot!

Cheers