FireBird is the most used open source database for Enterprise applications

FireBird Soars into Database Deployments, New Evans Data; Survey Open Source Databases Usurp Market Share

SANTA CRUZ, Calif.--(BUSINESS WIRE)--Jan. 18, 2005--FireBird is making major inroads in the database space, according to Evans Data Corporation's new Winter 2005 Database Development Survey. FireBird is the most used open source database for Enterprise applications, more database developers use FireBird for single purpose applications and FireBird is tied for the most used database for workgroup applications. Further, MySQL and FireBird are locked in a virtual tie in the open source database space with each being used by just over half of database developers who use open source databases

more reading this link
update:
better url on evens data site