As Arya carried him home, she talked softly to him as he lay in his egg, and upon entering the wards of Ellesmera, Firnen hatched for Arya, who became his Rider.
For the next six months, he was kept secret from everyone outside Ellesmera, as there were many who might try to kill him. Arya, faithful to the memory of the deceased Rider Oromis and his dragon, Glaedr, raised him on the Crags of Tel'naeir, where they had once lived. However, when Firnen wished to see more of the outside world, Arya could not resist his desire. Arya then took Firnen to meet Eragon and Saphira, her friends and allies. Upon meeting Saphira, Firnen and she became mates after completing a courtship ritual. They would remain together until she and Eragon left Alagaesia, which pained them both. After, it is assumed that Firnen and Arya helped train new Dragon Riders and Dragons in Alagaesia before sending them to study with Eragon and Saphira.
Firnen was described as being proud, like a young stag, but due to being raised away from anyone and everyone except Arya and animals for a longer time than Saphira, he was very unsure of what he could do until he met Saphira in person, when she tested him for mating by fighting with him. He was described as having a sunlit, happy mind. Firnen's mental voice was also the deepest of all the dragons Eragon had ever met, surpassing Glaedr, Saphira, Thorn, and all the Eldunari in this regard.