  Most cosmological models for gamma-ray bursts invoke the production of a
``fireball'' in a compact region, as indicated by the short time variability of
the observed GRBs. The high density of $e^+e^-$ pairs in such fireballs
inevitably makes them opaque to gamma-rays and requires the gamma-ray emission
to take place only after the fireball has expanded and became optically thin to
gamma's. Many of the ``traditional'' scenarios explain the temporal variability
of GRBs as being formed by growing instabilities in the expanding fireball.
Here we explore this possiblity and examine its validity.
