I had a similar problem, although with a different error messages. I had to rename the package (amd64 -> x86_64) and forc the installation with
rpm -ivh --force --nodeps FirebirdSS-2.5.8.27089-0.x86_64.rpm and now it works OK on my system. The Firebird 2.5.8 rpm has some problems on Centos 7, which have been recorded in the tracker, but not yet resolved. There is also a Centos-specific RPM on Epel, but unfortunately it is currently broken as well.
