  In this paper, we derive in a much detail the formula of count rates, in
terms of the integral of time, of gamma-ray bursts in the framework of
fireballs, where the Doppler effect of the expanding fireball surface is the
key factor to be concerned. Effects arising from the limit of the time delay
due to the limited regions of the emitting areas in the fireball surface and
other factors are investigated. Our analysis shows that the formula of the
count rate of fireballs can be expressed as a function of $\tau $ which is the
observation time scale relative to the dynamical time scale of the fireball.
The profile of light curves of fireballs depends only on the relative time
scale, entirely independent of the real time scale and the real size of the
objects. It displays in detail how a cutoff tail, or a turn over, feature
(called a cutoff tail problem) in the decay phase of a light curve can be
formed. This feature is a consequence of a hot spot in the fireball surface,
moving towards the observer, and was observed in a few cases previously. By
performing fits to the count rate light curves of six sample sources, we show
how to obtain some physical parameters from the observed profile of the count
rate of GRBs. In addition, the analysis reveals that the Doppler effect of
fireballs could lead to a power law relationship between the $FWHM$ of pulses
and energy, which were observed previously by many authors.
