Hmm, I wonder if maybe the best way to do this would be to use MultiTransitionEffectL<> with EFFECT_USER1 or something. You would use it with a transition that creates one firefly at a location specified by EffectPosition<>. At the end of the transition (after a small random delay) you would run TrDoEffect<> to trigger another EFFECT_USER1 with a new random location.
Edit: This method would not be very efficient, so generating a large number of fireflies would potentially be very slow.