After setting .ui-jqgrid-bdiv to {oveflow-y: visible;}, I ended up observing that the more rows I have in the grid, the more discrepancy there is between the expected height of the grid and the actual height.

At that point I realized that my browser has zoom on. Setting zoom to 100% eliminated any height discrepancy.

Another observation: adding border-collapse:collapse to the table eliminates the descrepancy but results in some horizontal borders disappearing (two rows not separated by 1px line).

My hypothesis is that Firefox breaks the computations for the number of pixels taht borders take when zoom is in effect.