Firesign's rhymes with:
aerodyne's, airline's, airlines, airlines', allgemeines, baselines, bloodlines, calamine's, canines, caroline's, coastlines, columbines, concubines, confines, dateline's, datelines, deadlines, dickstein's, duberstein's, enterline's, esterline's, feinstein's, felines, frankenstein's, frontline's, goldstein's, grapevines, guidelines, headlines, hemlines, hotlines, huffines, jardine's, landmines, lifelines, moneyline's, nightline's, nightlines, online's, outlines, outshines, paradyne's, pipeline's, pipelines, pipelines', porcupines, saberbein's, sidelines, skylines, smithkline's, storylines, strandlines, sunshine's, teledyne's, teradyne's, twonshein's, underlines, valentine's, valentines, villines, weinstein's
Firesign's sounds like:
farrakhan's, ferguson's, firkins, foraging, forcing, foregoing, foreseeing, forging, forgings, forgoing, forking, forsaking, freaking, freezing, frequencies, frequency, frerking, fresenius, freshening, freshens, freshness, fresno's