Fíriel was the third child and only daughter of Ondoher, the 31st King of Gondor. She was wed to Prince Arvedui of Arthedain, and accordingly became queen of that land once her husband ascended to the throne.
In T.A. 1944, her father and her brothers Artamir and Faramir were killed in the Disaster of the Morannon, leaving Gondor’s throne vacated and without a direct heir. Ancient law of Númenor dictated that Fíriel would become the ruling queen, though such law had never been practiced since the island’s sinking. Arvedui drew upon this law―with his status as Fíriel‘s husband, and as an heir of Isildur, whom he argued never truly relinquished his royalty in Gondor―to make his own claim, which was swiftly denied. The Steward Pelendur and the Council of Gondor instead awarded the kingship to Fíriel’s third cousin Eärnil II.
Her fate after Arthedain’s destruction was never recorded, and her son Aranarth continued both the lines of Isildur and Anárion in the Chieftains of the Dúnedain―all the way up to Aragorn II’s successful claiming of Gondor’s throne and the reunification of the two kingdoms in T.A. 3019.