  In this article, we investigate the blow-up for local solutions to a
semilinear wave equation in the generalized Einstein - de Sitter spacetime with
nonlinearity of derivative type. More precisely, we consider a semilinear
damped wave equation with a time-dependent and not summable speed of
propagation and with a time-dependent coefficient for the linear damping term
with critical decay rate. We prove in this work that the results obtained in a
previous work, where the damping coefficient takes two particular values $0$ or
$2$, can be extended for any positive damping coefficient. In the blow-up case,
the upper bound of the exponent of the nonlinear term is given, and the
lifespan estimate of the global existence time is derived as well.
