- The definition of firmer is more solid, or more unyielding when pressed.
If one mattress is harder and more unyielding than the other, it is an example of a firmer mattress.
firmer definition by Webster's New World
designating a carpenter's chisel or gouge with a thin blade fixed in a handle
Origin: French fermoir, altered ; from formoir ; from former, to form
a firmer chisel or gouge