IPA: /ˈfɜːrmər/
KK: /ˈfɜrmər/
More solid or strong than something else; having a greater degree of firmness.
The new mattress is firmer than the old one, providing better support for my back.
Firmer → It is formed from "firm" (from Old French *ferme*, meaning strong or solid) and the suffix "-er" (meaning one who or that which). The word "firmer" means one that is more strong or solid than another.
Think of something that is 'strong' ('firm') and the '-er' indicating more of that quality — so firmer means more strong.