the easiest way would be to use a PIC, that way, you could specify the delay bewtween each output going high.
However, if you don't want to get into microcontrollers, there are other ways.
One that comes to mind, is to use many 4017s linked together, so that the first one cycles through all its outputs, then the next one does it, so you would have maybe 100 outputs going off in sequence, all with the same delay between them. If you made this delay 0.5 secs each (for example), you could have delays in multiples of 0.5.
For example, you want the second firework to go off 1 second after the first, so you connect it to the thrid output, because the seqence would take 0.5s to get to the second output, then another 0.5s to get to the third, totalling 1second.
I havn't explained it very well, do you understand what I am trying to say?
Tim