Firebird 2.5.5 Final for supported Ubuntu releases is uploaded into Firebird stable ppa

Here you can find the latest stable release Firebird 2.5.5 for supported Ubuntu releases:15.10 (Wily), 15.04 (Vivid), 14.04 (Trusty), 12.04 (Precise)
Follow the Firebird 2.5.x guide for usage and installation.