Ok... for those who are going to try the fireworks again and wants to try the black card method.... here's the tricky part which may need some dry runs to get acquainted with the drill before the firework happens..
Do some test shots on manual mode first to get the optimal exposure for the background... eg. if at ISO200 @ F11, it takes 8 seconds for the background to get exposed properly... during the fireworks, you need to count the time that the film/CCD is exposed for 8 seconds...
The drill for me goes like this...
... wait for big burst (you can usually see a tell tale shot that fires high up, you probably know it's going to burst into a big one)
... depress and hold on to trigger in bulb mode
... Count in your head "Firing .... 1 thousand ... 2 thousand ... 3 thousand"
... First big burst should end about here, cover lens with black cloth and stop counting
... watch for secondary mid level bursts, not the low level ones
... remove black cloth and continue counting
... "4 thousand ... 5 thousand ... 6 thousand"
... cover lens and wait for pause of fireworks
... remove black cloth and continue counting for remaining 2 seconds
... release trigger
Of course this is just 1 more complex scenario...
If co-ordination poses a slight problem you may get a friend to depress the trigger when you start and release when you shout "8 thousand" or whatever timing is optimal for exposure...
This of course is if you want to expose properly for the background scene.
Another scenario can go like this...
If I estimate each burst takes about 2.5 seconds and I want to get 2 bursts, 1 high level and 1 mid level, then I set my shutter to 5 seconds, adjust aperature accordingly to get the optimal exposure.
The counting will be simpler like
High level Burst ... "Firing.. 1 thousand... 2 thou..." ... cover ... mid level burst .. expose "..sand... 3 thousand ... 4 thousand ..." release trigger.