The fir'bolg responce to the Great War could be compared to the American responce after WWI. The horrors of that war and the amount of their people slain resulted in an isolationist policy. I suspect that had Alric not kicked soulblighter's arse so quickly they might have eventually come to the aid of the allies.
As for human archers being better, sili is just being silly. Though your average fir'bolg isnt exactly a master archer ever (probably just hunters or mb the aristocrats of the society) , the human archers still attend fir'bolg universities to learn from the masters who are all fir'bolg. I suspect that if you actually got to see one of these masters in action.. you'd soon change your mind about the superiority of human archers :) Also keep in mind that the fir'bolg have the bre'unor to deal with at home and rather not have to concentrate on foreighn wars.
-Milk Man