stand firm vs stand fast

stand firm and stand fast both are verbs.

Word	Noun	Adjective	Verb	Adverb
stand firm	No	No	Yes	No
stand fast	No	No	Yes	No

As verbs, stand firm and stand fast are synonyms defined as:

Other synonyms of stand firm include hold firm, stand pat.

stand firm (verb)	stand fast (verb)
refuse to abandon one's opinion or belief	refuse to abandon one's opinion or belief
stand up or offer resistance to somebody or something