坚定的
结实的
坚固的
严格的（firm的最高级）

Firm \Firm\, a. [Compar. {Firmer}; superl. {Firmest}.] [OE.
ferme, F. ferme, fr.L. firmus; cf. Skr. dharman support, law,
order, dh? to hold fast, carry. Cf. {Farm}, {Throne}.]
1. Fixed; hence, closely compressed; compact; substantial;
hard; solid; -- applied to the matter of bodies; as, firm
flesh; firm muscles, firm wood.

2. 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.

Under spread ensigns, moving nigh, in slow
But firm battalion. --Milton.

By one man's firm obediency fully tried. --Milton.

3. Solid; -- opposed to fluid; as, firm land.

4. Indicating firmness; as, a firm tread; a firm countenance.

Syn: Compact; dense; hard; solid; stanch; robust; strong;
sturdly; fixed; steady; resolute; constant.