From Latin 'firmus' (strong), from PIE *dher- (to hold) — same root behind confirm, affirm, and farm.
Having a solid, unyielding surface; strong and sure.
From Old French 'ferme' (strong, steadfast), from Latin 'firmus' (strong, enduring), from PIE *dher- (to hold). The business 'firm' comes via Italian 'firma' (signature). Key roots: *dher- (PIE: "to hold