←
Proto-Germanic *furhu (mention)←
Proto-Germanic *furhō (mention)←
Proto-West Germanic *furhiþi (inherited)←
Proto-Indo-European *perkʷ- (root)←
Proto-Indo-European *pérkus (inherited)←
Proto-Indo-European *perkʷ- (mention)→
English firth (inherited)→
English frith (inherited)→
English Firth (derived)→
Old English fyrhþ (link)→
Middle English frith (inherited)