Firebird 2.5.6 Final for supported Ubuntu releases is uploaded into Firebird 2.5 ppa

Here you can find the latest stable release Firebird 2.5 for supported Ubuntu releases:16.04 (Xenial),  14.04 (Trusty), 12.04 (Precise)
Follow the Firebird 2.5.x guide for usage and installation.

ps : Firebird 2.5.6 is already included in Yakkety Yak (Ubuntu 16.10)