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.
He was a strong advocate for the parochial system, insisted on imposition of hands as requisite for the validity of ordination, and denied the right of parents who would not submit to discipline to claim baptism for their children.
In church politics he followed Baxter, preferring a reformed episcopacy to either the Presbyterian or the Congregational model, but laying most stress on the need of a well-ordered parish.
He supported himself by medical practice; the neighbouring justices took care that he should not be molested, though he regularly held conventicles.
On 22 July 1672 Daniel Ray, who had been ejected from Ridgewell, took out licences qualifying him to use his house as a presbyterian meeting-place.
Ray moved away in 1673, and Firmin remained till his death in sole charge of the congregation.
[2] His works include: He wrote also in defence of some of the above, and in opposition to John Owen, Daniel Cawdry, Thomas Grantham, and others.