As the daughter of King Ondoher of Gondor, Fíriel was wed to Prince Arvedui of Arthedain (the successor to Arnor after its breakup) during a time when Arvedui's father Araphant made an alliance with Ondoher. This marked the first time that Arnor and Gondor had renewed contact after a long estrangement and silence.
During the Wainriders' assault on Gondor, Ondoher and his sons Artamir and Faramir were killed. This meant that according to the Law of Númenor, Fíriel was now the Ruling Queen. Arvedui used this, as well as the fact that he was a direct descendant of Isildur and she was a direct descendant of Anárion, as an argument to claim the crown of Gondor. This was refused by Pelendur, the Steward of Gondor who influenced the Ruling Council that governed the kingdom in the interim. They instead chose Eärnil, who though being still of the Royal House of Gondor was not in the line of direct succession; his great-grandfather Arciryas was the brother of Narmacil II, and Eärnil's great-great-grandfather was King Telumehtar Umbardacil. Eärnil won the crown, being highly popular because he had destroyed the Wainriders, and much of the populace of Gondor thought little of Arthedain as it was weak despite the lineage of its lords.
Fíriel's marriage to Arvedui proved fateful, as it reunited the lines of Isildur and Anárion, and because Anárion's line would have died out afterwards when Eärnil's son Eärnur left no heir. Through her, Aragorn II could claim descent from both the line of Isildur and of Anárion.