(...)
ln -s /opt/firebird/lib/libfbclient.so.2.5.6 /usr/lib/libfbclient.so.2.5.1
(this creates a symlink named libfbclient.so.2.5.1 which fpc still uses to the new libfbclient.so.2.5.6)
That's not a good idea: You're mixing 64bit and 32bit stuff. That's not good. And it should not work, unless libfbclient wasn't installed correctly.
Well, it does work, perfectly.
And somehow the libraries in /usr/lib64 are not found. The default Firebird installations installs them there so I guess they are installed correctly.
But... I thought the /usr/lib IS for 64bit libraries on a 64bit OS !!
https://bbs.archlinux.org/viewtopic.php?pid=815764#p815764Although I agree it would be better to separate 32bit and 64bit libraries it seems this is not always done and the /usr/lib is also used for 64bit libraries.
Edit: I though the libraries are always in /opt/firebird/lib/ and Firebird symlinked them to /usr/lib. (but in this case it was to /usr/lib64 for some reason)
PS. I used the tarball and used the install.sh because I was on Mint. Maybe that's the difference.