Between them they created a number of popular children's TV programmes, The Saga of Noggin the Nog, Ivor the Engine, Clangers, Bagpuss and Pogles' Wood.
[2] It was while he was teaching at Central School of Art that Oliver Postgate came looking for, as Firmin put it: "... someone to illustrate a television story – someone who was hard up and would do a lot of drawing for very little money".
Most of Smallfilms' animation work was produced in a barn on Firmin's land in Blean near Canterbury in Kent.
Firmin made the sets, puppets and backdrops for the programmes, often also contributing to making sound and visual effects during filming.
In 1959, with his wife Joan, he devised a programme of nursery rhymes for Associated-Rediffusion, called The Musical Box, which used live cardboard animation and puppets.
[9] In 1994, Firmin provided an illustration for a British postage stamp, SG1804, featuring characters from Noggin the Nog.
[1] Between 12 May and 29 July 2018, Firmin's work was featured in the exhibition Clangers, Bagpuss & Co, organised by the V&A Museum of Childhood at the Ferens Art Gallery in Hull.