The bone marrow charity, which helps to find donors for people in need of bone marrow transplants, will receive a year's worth of free social media support from the digital specialist.
The agency launched its 2010 pro bono scheme in partnership with PRWeek in May and received 18 submissions.
Firefly then sat down with PRWeek to mark the entries using strict criteria. Three charities were selected for 'chemistry meetings' and The Anthony Nolan Trust was finally selected after an agency-wide vote.
The details of the charities that were not successful in Firefly's scheme have been passed on to the PRCA.
PRCA chairman Francis Ingham said he was in the process of setting up a matchmaking scheme for charities that needed pro bono support. 'We will use our existing business referrals services to get clients and agencies matched,' he said. Ingham also urged charities to get in touch.
Firefly will now create and manage a social media campaign to get more 18- to 25-year-olds to sign up to The Anthony Nolan Trust's bone marrow donor register. Instead of giving a blood sample, donors are now able to register by giving a spit sample.
The charity's director of comms David Knights said: 'We need to fight misunderstanding that donating bone marrow is a painful and invasive process. In fact, more than 80 per cent of people give in a similar way to giving blood.
'Secondly, some people, particularly those under 45 years old, have never heard of us. Our main message is to tell people that if you register, you can save someone's life.'
Firefly's CEO Claire Walker said the agency's selection criteria was similar to that of a client pitch. She added: 'We chose The Anthony Nolan Trust because it is at a stage where we felt we could make a big difference.'
She said the agency had learned lessons after seeing the pitch process from a client's perspective.
'We will allow (our) individual personalities to come through more in future pitches,' she added.