this device shoots off 23040 fireworks in 7 minutes 46 seconds and is preloaded
designed this myself took about 2 hours on figuring out how to lay the redstone so the clock was timed right.
you may adjust the repeaters to change the timings but it isn't recommended because doing so may lag your server (the combination of the redstone clock and the fireworks can put stress on your server).
ROTATING THE SCHEMATIC NOT RECOMMENDED DUE TO DISPENSERS NOT ALIGNING RIGHT WHEN DOING SO
i recommend using a plugin called RandomFireworks to fill the dispensers.
bug in worldedit causes dispensers to come out of alignment you may need to realign the dispensers to dispense into the lava.

make sure dispensers are facing right direction or you will not get proper results. worldedit has issues placing any redstone device in proper position. the dispenser must face the lava

Additional Notes

you need at least 1gb ram on your server to run this device.
also because this uses a redstone clock make sure you power it down when finished or you could cause other players to lag in the area if left running for long periods of time.