select UUID_TO_CHAR(gen_uuid()) from rdb$database

Gera chave unica no formato:
434683BA-9355-164A-9695-E68709ADA37E
668068DA-3744-4A4D-B5CF-694B18245AA5
A8B459BF-C224-BA43-8BD0-CA10EA8DA6EE
E9B0D0C7-E735-3E42-B7A0-1CE776D338AE

Referências para consulta:
http://www.firebirdsql.org/refdocs/langrefupd25-intfunc-gen_uuid.html
http://www.firebirdsql.org/refdocs/langrefupd25-intfunc-uuid_to_char.html