Looks like Mozilla is playing the version # game. You know, pump up the primary (part of # before the .) version # to keep up/catch up with other browsers with higher version #'s.
Otherwise, the updates in FF 5 look to me like they should have been 4.0.2, or some continuation within the 4 series, in the same manner as the 3 series, which ended in 3.6.17, as I recall.