  The jet composition and radiative efficiency of GRBs are poorly constrained
from the data. If the jet composition is matter-dominated (i.e. a fireball),
the GRB prompt emission spectra would include a dominant thermal component
originating from the fireball photosphere, and a non-thermal component
presumably originating from internal shocks whose radii are greater than the
photosphere radius. We propose a method to directly dissect the GRB fireball
energy budget into three components and measure their values by combining the
prompt emission and early afterglow data. The measured parameters include the
initial dimensionless specific enthalpy density ($\eta$), bulk Lorentz factors
at the photosphere radius ($\Gamma_{\rm ph}$) and before fireball deceleration
($\Gamma_0$), the amount of mass loading ($M$), as well as the GRB radiative
efficiency ($\eta_\gamma$). All the parameters can be derived from the data for
a GRB with a dominant thermal spectral component, a deceleration bump feature
in the early afterglow lightcurve, and a measured redshift. The results only
weakly depend on the density $n$ of the interstellar medium when the
composition ${\cal Y}$ parameter (typically unity) is specified.
