He matriculated at Emmanuel College, Cambridge, in December 1629, his tutor being Thomas Hill. At Cambridge he studied medicine. In 1632 he went with his father to New England. While at Boston, Massachusetts, he was ordained deacon of the first church, of which John Cotton was minister. At Ipswich, Massachusetts, he received in 1638 a grant of  of land. He practised medicine in New England, and was reputed a good anatomist. He married there Susanna, daughter of Nathaniel Ward, pastor of the church at Ipswich. About 1647 he returned to England, leaving wife and family in America; on the way he was shipwrecked on the coast of Spain. In 1648 Firmin was appointed to the vicarage of Shalford, Essex, which had been vacant a year since the removal of Ralph Hilles to Pattiswick. At Shalford he was ordained a presbyter by Stephen Marshall and others.
They matriculated at Emmanuel College, Cambridge, in December 1629, their tutor being Thomas Hill. At Cambridge they studied medicine. In 1632 they went with their father to New England. While at Boston, Massachusetts, they were ordained deacon of the first church, of which John Cotton was minister. At Ipswich, Massachusetts, they received in 1638 a grant of land. They practiced medicine in New England and was reputed a good anatomist. They married there Susanna, daughter of Nathaniel Ward, pastor of the church at Ipswich. About 1647 they returned to England, leaving wife and family in America; on the way they were shipwrecked on the coast of Spain. In 1648 Firmin was appointed to the vicarage of Shalford, Essex, which had been vacant a year since the removal of Ralph Hilles to Pattiswick. At Shalford they were ordained a presbyter by Stephen Marshall and others.