She is notable for illustrating the book covers of Alexander McCall Smith's The No.
Firmin is the second daughter of Peter Firmin, co-creator with the late Oliver Postgate, of the children's television programmes Pingwings, Pogles' Wood, Noggin the Nog, Ivor the Engine, Clangers and Bagpuss.
Since then she has worked as a freelance illustrator for books, magazines, newspapers and for advertising and design companies.
She has illustrated the covers for the ten books in the series, to critical acclaim.
With McCall Smith, Firmin has supported the literacy charity Book Aid International, donating an illustrated McCall Smith story for auction.