Yeah, what everyone else has already said. I find that ~4 seconds is best for fireworks. That and ISO100 and aperature anywhere from f5-8. And then flash to match.
And you'll probably need to shoot wider than you think. I went to a firworks show with the 24-70 on my 5D2 and it worked, but I was wishing I brought the 16-35 instead.
Not sure the sillouette idea will work as the firworks will probably not create enough backdrop, but maybe. Worth a shot, but don't risk getting the flash shot trying for the sillouette (IMO).