[1] He joined the crucible steel works of Sanderson Brothers where his father worked as head smelter, but left in 1842 to set up his own business with his brother, Thomas Jr. Their father joined them shortly afterwards, and in 1852 Thomas Firth & Sons had expanded into larger premises at the Norfolk Works in Savile Street, which had the largest rolling mill in Sheffield.

The Firth's business expanded into the armaments market, installing two large steam hammers in 1863.

He also built a mansion for himself on the outskirts of Sheffield at Oakbrook, Ranmoor, now part of Notre Dame High School.

In 1879, he opened Firth College to teach arts and science subjects, which later became part of the University of Sheffield.

[2] On 16 November 1880 Firth was at his Norfolk Works when he suffered a stroke, and died 12 days later.