On the resignation of Roger Pickering, around 1752, he became in addition one of the two preachers of the Sunday evening lecture at Salters' Hall.
From October 1769 to January 1775 he was relieved of the afternoon service on his lecture evenings by Samuel Morton Savage, D.D.
As a member of the Coward Trust he had much to do with the revised plan of education adopted by the trustees on Philip Doddridge's death.
This speech was reported, without notes, by Furneaux with assistance from another hearer, Samuel Wilton, D.D., independent minister of the Weighhouse, Eastcheap.
In 1769 the fourth volume of William Blackstone's Commentaries on the Laws of England appeared, in which, under the head of 'Offences against God and Religion,’ nonconformity is treated as a crime.
In the following year Furneaux published his 'Letters to Mr. Justice Blackstone,’ with a moral argument against enforcing religious truths by civil penalties.
In the course of the debate the remark was made by Lord North, who opposed the petition, that if similar relief were asked by the dissenting clergy there would be no reasonable objection to it.
In 1771 Furneaux was engaged in transcribing and editing the biblical annotations of Samuel Chandler, but the work was never published.