Just add the following to the .cfg;
[smokesystem]
smoke.0=0.00, -15000.00, 00.00, fx_firework12
smoke.1=0.00, -15000.00, 00.00, FX_firework13
You can change the -15000.00 to move the effect closer or farther out, but a bit less than three miles looks about right. It takes a bit of practice to get the impacts to all end up on the ground and in one spot.
