The agency said Firth (pictured) will "provide creative direction" across Speed’s three divisions: consumer & lifestyle, business & corporate, and food & hospitality.
She was most recently director of strategy and planning at TVC. Her previous roles include creative director at Lexis PR.
Speed MD Kelly Pepworth said: "Sarah has a fantastic creative pedigree of delivering award-winning, impactful campaigns. She will be working closely with all our divisions to ensure we are constantly challenging ourselves to bring best in class thinking that excites our clients, engages their audiences and inspires influencers."
Speed is listed at 62 in the most recent PRWeek UK Top 150 Consultancies table, with revenue of £4.8m and headcount of 71 in 2016. Recent account wins at Speed include water filtration specialist BRITA Professional.
This morning saw another well-established PR agency - M&C Saatchi PR - announce its first creative director.