DATASUS using Firebird
Firebird is being used (and distributed) in several applications produced by DataSUS. SUS is the name of the unified public health system in Brazil. DataSUS is the department responsible for the development of computer applications, etc.
The applications are used in hospitals spreaded in the whole country. You can find several references of Firebird in DataSUS site, like here and here. Probably there are thousands of Firebird servers running only in the Brazilian’s hospitals.