WordNet sense 1 (marked by firm determination or resolution; not shakable):

firm, steady, stiff, unfaltering, unshakable, unwavering, unbendable

WordNet sense 2 (firm in purpose or belief; characterized by firmness and determination):

resolute

WordNet sense 3 (firm and dependable especially in loyalty):

staunch, unswerving

WordNet sense 4 (steadfast in purpose or devotion or affection):

constant