adjective

having a solid, almost unyielding surface or structure.

the bed should be reasonably firm, but not too hard

hard,solid,unyielding,resistant,solidified,hardened,compacted,compressed,condensed,dense,close-grained,stiff,rigid,inflexible,inelastic,walkable,congealed,frozen,set,gelled,stony,steely,adamantine

verb

make more solid or resilient.

how can I firm up a sagging bustline?

adverb

in a resolute and determined manner.

the Chancellor has held firm to tough economic policies