In 1972 he also starred in the ITV (London Weekend Television) series The Adventures of Black Beauty, episode "The Runaway", playing David Abbott.

In July 1973, Firth appeared at Laurence Olivier's National Theatre,[3] starring in the stage version of Peter Shaffer's play Equus as a teenager being treated by a psychiatrist, and in October 1974 repeated the role in the Broadway production, receiving a Tony Award nomination for his performance as Alan Strang.

Firth's first major role as an adult was the title part in a BBC Television Play of the Month adaptation of Oscar Wilde's novel The Picture of Dorian Gray (1976).

That same year saw the release of the World War I film Aces High which featured Firth as the inexperienced RFC pilot Lt. Stephen Croft.

He also portrayed the Emperor Vespasian in "The Jewish Revolt" episode of the BBC series Ancient Rome: The Rise and Fall of an Empire.

In 2014, Firth played the part of the character Andrew Rawlins in Undeniable, a two-part series made for the ITV network.

He has been responsible for performances reading Pat Barker's Regeneration, The Ghost Road and The Eye in the Door, Suspicion by Robert McCrum, Maurice by E. M. Forster, Brave New World by Aldous Huxley, Sebastian Faulks' Birdsong and Thomas Hardy's Tess of the d'Urbervilles.