Firbolg

As an altered race, Firbolg mostly stay true to their descriptions provided in the source books of 5e. The sourcebooks (Volo's in this case) tell you everything you need to know to roleplay a Firbolg, how their societies work and how they see themselves as natures guardians.

True Fey:
Much like the Gnomes of Nevandaar, the Firbolg are Fey creatures, which explain their affinity for nature and how they can communicate to plants and animals.

Firbolg were made from the thoughts of a single-selfless Giant who, during the age of Dragons, protected a forest from a particularly malicious Black Dragon. He gave his life to protect its inhabitants, killing the dragon in the process. As the Feywild is an echo of the Prime Material, his act had small repercussions in the Feywild, and where the Giant fell - clutching into a tree to support himself in his last few moments, Firbolg were created in the Feywild.

Protectors of the forest:
Firbolg however, unlike the Gnomes, wished to leave the Feywild the moment they were born. Things in the Feywild, most things, respected natural order. All hailing from Fey forests, or woodlands. The Firbolg saw the Material Plane's forests and woods as requiring protection, as they had no guardians like they did in the Feywild.

Whilst the Gnomes established Kingdoms in the Feywild, the Firbolg journeyed into the Prime Material, a combination of Giants, Fey, and a protective mindset akin to a Warden. Today they act much the same as described in the sourcebooks of 5e. They have small communities dotting many of the heavily wooded areas of Nevandaar, from the woodlands of Immria to the Jungles of Kakum.