be a firm believer in (something)
To have a strong conviction that something (stated after "in") is important or worthwhile. I'm a firm believer in resting on the weekends so that I don't get burned out.
be a great/firm believer in something
believe strongly that something is good, important or valuable: My mother was a great believer in horoscopes all her life.The boss was a firm believer in developing strong teamwork.