Collocations for firm - adj.

be, feel, look, seem | remain, stay
Exercise is important if you want your muscles to stay firm.

make sth
Use extra stuffing to make the cushions firmer.

keep sth
exercises to keep your muscles firm

very | fairly, quite, reasonably

be, sound | hold, remain, stand
Jo held firm: nothing else would do. | We stand firm on these principles.

very | fairly, quite

with
I have always been quite firm with my children.