/usr/lib/firebird/2.5/firebird.conf is in firebird2.5-common 2.5.3.26778.ds4-5+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/firebird/2.5/firebird.conf
/etc/firebird/2.5/firebird.conf