A youth academy graduate of Defensor Sporting, Fiermarin joined newly promoted side Montevideo City Torque on loan for 2018 season.
[1] On 22 July 2021, Belgian club Lommel announced the signing of Fiermarin on a season long loan deal.
[3] In June 2024, he signed a four-year contract with Defensa y Justicia after the club bought 60% of his economic rights.
[4] In January 2025, Fiermarin joined Colombian club Deportes Tolima on a season long loan deal.
[7] However he didn't play any match in the tournament as he was third-choice goalkeeper behind Renzo Rodríguez and Francisco Tinaglini.