Seems even if you only launch one firework at a time and wait for the boom, after 3 or so rockets the client will still hang.
This will happen with the default "Firework star" that has no color or properties assigned. You cant spawn it with commands (that I know of) or use them straight from the creative inventory.
Definitely not fixed in 1.4.5 pre-rel.
I have seen this happen with players too and not just specific to door blocks.
Flowing lava does not have an effect on the fireworks.
While I have seen it happen a couple of times with steps and halfstep blocks, I cannot reliably reproduce it.