<<Sep 23 16:30:43 mac-mbp Installer[66897]: Set authorization level to root for session
Sep 23 16:30:43 mac-mbp Installer[66897]: Administrator authorization granted.
Sep 23 16:30:43 mac-mbp Installer[66897]: Package file://localhost/Users/macuser/Downloads/FirebirdCS-2.5.4-26856-x86_64.pkg not
supported by PK
Sep 23 16:30:43 mac-mbp Installer[66897]: Unable to use PK session due to incompatible packages. Terminating.>>

I suspect its not SIP, but the fact that we use an outdated package format. Looks like El Capitan has finally deprecated it.
I found this https://forums.developer.apple.com/thread/8676
Note the error message, they then switched from /var to /Applications and all was ok.
While we use /Library/Frameworks

Regards
Paul