Stiffener rhymes with:
oftener, softener, aachener, airconditioner, almoner, commissioner, commoner, conditioner, confectioner, coroner, examiner, executioner, falconer, fastener, faulconer, foreigner, freshener, gardener, gissendanner, hardener, heavener, kamenar, kitchener, laminer, listener, londoner, mariner, masoner, oftener, opener, parishioner, pensioner, petitioner, practitioner, prisoner, probationer, questioner, quintanar, reasoner, rosener, schuldener, softener, stationer, sweetener, thickener, tichenor, usinor, vacationer, vanwagoner, wagener, waggener, waggoner, wagoner, warriner