This came to me the other day...
Ok, the Fir'Bolg say the hell with the Legion after the war with Balor and disappear back into the Ermine, although they still train human archers. THis tells us that 1.) Humans had no archery prior to THIS war (perhaps explaining also the lack of cavalry in the legion, they just haven't developed it to a non-experimental stage).
The Fir'Bolg claimed that they went back to defend their homes from the Bre'unor, BUT...they've been fighting the skull-faced cultists for a damn long while now. So one wonders WHY they disappeared back home and didn't even ask for some help.
Perhaps the Fir'Bolg fear that they will become as the Myrmidons of the next age if they stay too close to the Legion and the future potential leveller?
Perhaps they are plotting something themselves?
Perhaps the Bre'Unor have increased in power and number?
I dunno :)