He created the animated web series Salad Fingers (2004–present) and co-wrote the horror comedy film Kuso (2017).

[16] Firth also co-created the comedy rapper character MC Devvo, a dimwitted chav performed by his friend Christian Webb, for a series of videos that satirised the music scene of Doncaster's white working class.

[20][21] In 2014, he created the animated music video for the Flying Lotus song "Ready Err Not", which Pitchfork described as disturbing and highly graphic.

[22] Firth also co-wrote the 2017 body horror feature film Kuso, which was co-written and directed by Flying Lotus.

[7] Firth says that he "[takes] inspiration from the unpredictability of dreams", and describes applying this style as difficult because of the lack of spontaneity in producing animation.