Tim Bowen provides a firm grounding in collocations.

In the sense of ‘definite and not likely to change’, the adjective firm collocates with several groups of nouns. There can be a firm basis for something and other words with a similar meaning include base, footing and foundation, as in ‘Many people support the view that civil partnerships would contribute to their relationships being on a much firmer footing’.

Firm also collocates with a number of words that indicate a person who believes in something, such as advocate, believer and supporter, as in ‘She has always been a firm believer in the principle that a good education can open the door to future career opportunities’.

Feelings or beliefs can also be firm and other words in this category include commitment, conviction, resolve and stance, as in ‘He likes to portray himself as a man of firm moral convictions but, above all, as someone who is entirely trustworthy’ or ‘The government intends to take a firm stance on this issue’.

Firm can also be used with the noun evidence, as in ‘As yet the police have no firm evidence that the principal suspect actually perpetrated the offence but say they are following several lines of enquiry’.

An understanding of a particular subject or issue can also be firm and the most frequently used nouns in this category are grasp and grounding, as in ‘To be successful in this business you need a firm grasp of civil engineering’ or ‘The programme aims to give young people a firm grounding in the practicalities of business’. The adjective solid can replace firm in both of these examples.