Born and raised in Yorkshire, the blond Firth began performing in amateur productions and by the time he was in his teens had dropped out of school to pursue an acting career. He landed a role as one of "The Double Deckers", a group of seven kids who cope with problems, that aired first in the UK and later in the US (ABC, 1970-72). Firth later gained fame in England as one of "The Flaxton Boys" (BBC, 1971-72) before segueing to the stage and screen. He made a brief appearance in Franco Zeffirelli's "Brother Sun, Sister Moon" (1973) the same year he created his acclaimed stage role in "Equus". Firth spent the 1974 season with the National Theatre Company appearing in "Spring Awakening", "Measure for Measure" and "Romeo and Juliet".

In 1976, Firth had his first major film role, as a rookie WWI fighter pilot in "Aces High". He followed with the title role in Tony Richardson's uneven "Joseph Andrews" (1977). After his Oscar-nominated turn in "Equus", he essayed yet another troubled youth in "When You Coming Back Red Ryder?" before returning to period garb as the cleric's son who marries and abandons "Tess" (both 1979). Most of his film roles in the early 1980s generally did not provide Firth with much opportunity to shine. One of his better chances came with "Letter to Brezhnev" (1985), in which he was a Russian sailor romancing a Liverpudlian girl. He again spoke Russian as the doomed sailor in John McTiernan's "The Hunt For Red October" (1990). Firth was sympathetic as the doctor treating Joy Gresham (Debra Winger) in Richard Attenborough's "Shadowlands" (1992) and excelled as the nasty stage manager in Mike Newell's "An Awfully Big Adventure" (1995).