Fittingly, for these kinds of mysterious creatures, hardly any is known about firbolgs. They originated in Irish mythology as founding settlers and before D&D editions ended up somewhat faithful to this, portraying firbolgs as powerful Celtic warriors. This can be a considerably cry although, from the firbolgs of 5E. Tundra: https://gnomewizards01245.onesmablog.com/a-secret-weapon-for-undead-tabaxi-68140537