In this paper, we generalize a previously developed model describing the behavior of optical bistability in the operation of a quasi-three-level laser by taking into account a Gaussian transverse distribution for the laser mode and including inhomogeneous line broadening for the laser medium. The results show that apart from some quantitative modifications, the primary characteristics of bistability predicted by the simplified model remain correct; in particular, the fundamental condition for the existence of bistability, , turns out to hold universally. It is also revealed that in comparison with the results obtained in the simplified model, the intensities at the up- and down-threshold will be increased, whereas the bistability range will be reduced.