/usr/lib/x86_64-linux-gnu/libfbclient.so is in firebird-dev 2.5.3.26778.ds4-5+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfbclient.so.2.5.3
/usr/lib/x86_64-linux-gnu/libfbclient.so.2.5.3