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.
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.