Gamma-ray bursts (GRBs) are believed to originate from ultra-relativistic fireballs, with initial Lorentz factor eta similar to10(2) - 10(3). in order to avoid that the MeV photons are absorbed for e(+/-) production (Meszaros 2002). However very high energy photons may still suffer from gammagamma interaction. We show here that in a wide range of model parameters, the resulting pairs may dominate electrons associated with the fireball baryons. Because the pairs do not suffer from annihilation for an observed GRB, they would be carried into a reverse shock so that a shocked pair-rich fireball may produce a strong flash at lower frequencies, say, the IR band, in contrast with optical/UV emission from a pair-poor fireball. This may provide an explanation for the rarity of prompt optical detections. A rapid response to the GRB trigger at the IR band would detect such a strong flash.