How to install/upgrade Firebird manually on El Capitan

Background: The current Firebird automatic installer on El Capitan is no longer working, its format has been
finally deprecated. The installer is currently undergoing a complete re-write to comply with the Flat Package Format. Until this work is completed, you can manually install Firebird using the following method.

Classic

Download FirebirdCS-2.5.4-26856-x86_64.pkg.zip
Unzip it
cd FirebirdCS-2.5.4-26856-x86_64.pkg
cd Contents
gunzip Archive.pax
pax -r -f Archive.pax (or unpack the file using Finder)
as sudo
cp -r Firebird.framework /Library/Frameworks/Firebird.framework
cd Resources
./postinstall

If you are upgrading from a previous version use:
./preupgrade
and then
./postupgrade

SuperServer

Download FirebirdSS-2.5.4-26856-x86_64.pkg.zip
Then follow the same steps as above, and ignore any errror messages that reference
a, /Resources/doc/doc: No such file or directory
b, chmod /Library/StartupItems/Firebird/Firebird: No such file or directory
c, /Library/LaunchDemons/org.firebird.gds.plist: service is already loaded.

a ps -eaf should show both fbserver and fbguard running.