  Fireballs are an essential part of any cosmological $\gamma$-ray burst. We
drive here a stability criterion for fireballs and show that fireballs are
Rayleigh-Taylor unstable in any region in which the entropy decreases outward.
The instability begins to operate when the fireball becomes matter dominated.
Among the possible implication of the instability are: (i) Conversion of a
fraction of the radiation energy to a convective energy expressed in the motion
of bubbles relative to each other. (ii) Penetration of fast bubbles through
slower ones and creation of high $\gamma$ regimes which are essential for
efficient conversion of the energy to $\gamma$-rays. (iii) Formation of rapid
variation (of the scale of the bubbles) in the observed $\gamma$-rays.
