Firmer rhymes with:
dermer, ermer, germer, murmur, schermer, schirmer, shermer, stermer, sturmer, termer, vanwormer, wermer
Firmer sounds like:
fahrner, farmer, farner, ferner, fernery, forerunner, former, forner, fournier, framer, freemyer, frohnmayer, fromer, frommer, frymire, furnari, furner