<<Having identified and fixed a problem in file_params.h and Info.plist, we
now have a working prototype installer via pkgbuild, albeit very alpha, but
its installing and working OK for me on El Capitan.

Hopefully I should have something constructive early next week.>>

And are now available for download.
http://sourceforge.net/projects/firebird/files/firebird-MacOS-X_darwin/2.5.4-Release/

Regards
Paul