From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Βέβαιος (Strong'S #949 — Adjective — bebaios — beb'-ah-yos )

"firm, steadfast, secure" (from baino, "to go"), is translated "firm" in  Hebrews 3:6 , of the maintenance of the boldness of the believer's hope, and in  Hebrews 3:14 , RV, of "the beginning of our confidence" (AV, "steadfast"). See Steadfast , Sure.

2: Στερεός (Strong'S #4731 — Adjective — stereos — ster-eh-os' )

"solid, hard, stiff," is translated "firm" in  2—Timothy 2:19 , RV, "the firm (foundation of God)," AV, "(standeth) sure;" stereos is not part of the predicate; "solid (food)" in  Hebrews 5:12,14 , RV; "steadfast" in  1—Peter 5:9 . See Solid , Steadfast , Strong.

 Acts 3:7,16 16:5 Colossians 2:5

Webster's Dictionary [2]

(1): ( superl.) Solid; - opposed to fluid; as, firm land.

(2): ( a.) The name, title, or style, under which a company transacts business; a partnership of two or more persons; a commercial house; as, the firm of Hope & Co.

(3): ( a.) To fix; to settle; to confirm; to establish.

(4): ( superl.) Not easily excited or disturbed; unchanging in purpose; fixed; steady; constant; stable; unshaken; not easily changed in feelings or will; strong; as, a firm believer; a firm friend; a firm adherent.

(5): ( a.) To fix or direct with firmness.

(6): ( superl.) Indicating firmness; as, a firm tread; a firm countenance.

(7): ( superl.) Fixed; hence, closely compressed; compact; substantial; hard; solid; - applied to the matter of bodies; as, firm flesh; firm muscles, firm wood.

King James Dictionary [3]

FIRM, a. ferm. L. firmus. This is the root of L. ferrum, iron.

1. Properly, fixed hence, applied to the matter of bodies, it signifies closely compressed compact hard solid as firm flesh firm muscles some species of wood are more firm than others a cloth of firm texture. 2. Fixed steady constant stable unshaken not easily moved as a firm believer a firm friend a firm adherent or supporter a firm man, or a man of firm resolution. 3. Solid not giving way opposed to fluid as firm land.

FIRM, n. ferm. A partnership or house or the name or title under which a company transact business as the firm of Hope & Co.

FIRM, ferm. L. firmo. To fix to settle to confirm to establish.

And Jove has firm'd it with an awful nod.

This word is rarely used, except in poetry. In prose, we use confirm.