She has a firm belief that honesty is the best policy.

Strongly held or determined; not easily changed.
He has a firm grip on the rope.
Her decision was firm and final.
They have a firm commitment to quality.
firm stance
firm belief
firm handshake
steadfast
resolute
unwavering
The baker will firm the dough by kneading it.

To make more solid or secure.
Firm the jelly before serving.
He will firm his stance on the issue.
Let the pudding firm in the fridge.
firm up
firm the decision
firm the ground
harden
strengthen
set