There was a post to the Firebird Support list, asking how you link Qt to the libfbclient.dylib on MacOSX Firebird SuperServer. The problem being, that libfbclient.dylib is not "shipped" as part of the MacOSX framework... heres the detailed answer on how it works.
Firebird on MacOSX is delivered as a framework,
For more information on a frameworks look on : http://developer.apple.com
In a nutshell - "A framework is a hierarchical directory that encapsulates shared resources, such as a dynamic shared library, nib files, image files, localized strings, header files, and reference documentation in a single package. Multiple applications can use all of these resources simultaneously. The system loads them into memory as needed and shares the one copy of the resource among all applications whenever possible."
This is the "best" way to deliver an application on MacOSX.
However to do this the main dylibs (shared libraries) that run Firebird become part of the framework itself.
The main dylib that drives Classic is libfbembed, for SuperServer its libfbclient.
With Classic we ship both libfbembed and libfbclient, libfbclient as a separate dylib can be found in the libraries directory.
for SuperServer we ship libfbclient
However libfbembed (Classic) and libfbclient (SuperServer) are not directly obvious as shared libraries, i.e. if you look for them by that name with a dylib extension you are not going to find them. However if you took a close look at postfix.darwin in the Firebird source builds/posix (this is the code that sets up the framework) you will find that they become Firebird.framework/Firebird which is a link to Firebird.framework/Versions/Current/Firebird.
Where Firebird.framework/Firebird is libfbembed.dylib for Classic and
Firebird.framework/Firebird is libfbclient.dylib for SuperServer