Firth rhymes with:
furth, berth, birth, dearth, earth, fuerth, furth, gerth, girth, hirth, kerth, kurth, mirth, perth, rebirth, unearth, wentworth, werth, wirth, worth, wurth
Firth sounds like:
faherty, faraday, fared, farhat, fart, faubert, faurot, favored, favorite, fayard, feared, ferd, ferdie, ferret, ferreted, ferretti, ferried, ferrite, fertitta, fevered, fibroid, fioretti, fiorito, fired, fireweed, firewood, foard, forayed, ford, forde, forehead, foret, foreword, fort, forte, forth, forthwith, forti, fortieth, fortitude, forty, forward, forwarded, forwood, fourth, fourtou, fradette, frady, frate, fraud, frayed, fread, fred, freda, freddie, freddy, freddye, frede, fredette, fredo, freed, freet, freid, freidy, freiheit, fret, frett, fretted, freud, frida, friday, fried, frieda, friede, friedt, frith, frito, froth, frothy, fruit, fruity, fruth, fuerth, furrowed, furtado, furtaw, furth, furuta