Geographical location: Most firbolgs live in groups, secluded from the rest of the world.
Common physical attributes: Rather bulky and tall, and have a cow-like appearance.
Religion: Worship nature, and thus many firbolgs are druids
Lifestyle: Simple lifestyle in the woods.
Overall ideals/morals: They tend to be exclusionary and isolated.
Family: They live in tight-knit tribes or clans in the wilderness.
Language: Most if not all can speak common and giant.