I did it in the park (Epcot) with my mini-not-so-stable tripod, while it was pouring down rain, so here's my best attempt at what works.
f8-16 on bulb, ISO200, 320. Leave the shutter open and cover the lens with the cap (or your hand, easier), expose to light only when the fireworks goes up, you get multiple fireworks on a single frame and it looks awesome! Half way through the cheap ball head snapped apart, and I was left to bracing myself against a railing.

The underexposed part on the left is the top-notch $7 poncho the Disney folks will sell you.