Installed the official binary http://prdownloads.sourceforge.net/firebird/Firebird-CS-1.5.1-MacOS.zip libgds.so is actually /Library/Frameworks/Firebird.framework/Versions/A/Firebird
A script (http://www.b-shock.org/macosx/firebird/) that gives a standard Firebird install:
#!/bin/bash
mkdir /usr/local/firebird
cd /usr/local/firebird
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Headers ./include
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Resources/bin ./bin
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Resources/examples ./examples
mkdir lib
cd lib
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Firebird ./libgds.a
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Firebird ./libgds.so
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Firebird ./libgds.so.0
ln -s /Library/Frameworks/Firebird.framework/Versions/A/Firebird ./libib_util.so
cd /usr/local/include
ln -s /usr/local/firebird/include/* .
cd /usr/local/lib
ln -s /usr/local/firebird/lib/* .
exit