Hello Paul,

I haven't actually confirmed that this is a SIP problem. I was suspecting this after googling a bit with log message received from installer logs. I have yet to disable SIP altogether, but will try this later today.

Included is the installer log created by Firebird Installer.

Sep 23 16:30:02 mac-mbp Installer[66897]: LSExceptions [0x7f93d9603070] loaded
Sep 23 16:30:02 mac-mbp Installer[66897]: @(#)PROGRAM:Install  PROJECT:Install-1000
Sep 23 16:30:02 mac-mbp Installer[66897]: @(#)PROGRAM:Installer  PROJECT:Installer-853
Sep 23 16:30:02 mac-mbp Installer[66897]: Hardware: MacBookPro9,1 @ 2.60 GHz (x 8), 8192 MB RAM
Sep 23 16:30:02 mac-mbp Installer[66897]: Running OS Build: Mac OS X 10.11.1 (15B17c)
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: TMPDIR=/var/folders/z1/c5tfsznx0ts_cdbdhww69qrm0000gn/T/
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: FS_TRASHER_SOCKET=/private/tmp/com.apple.launchd.9wJ4tCTvtL/ListeningSocket
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: __CF_USER_TEXT_ENCODING=0x1F5:0x0:0x0
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: HOME=/Users/macuser
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: SHELL=/bin/bash
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.OR4quis6LU/Render
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.j4vOrTOuXP/Listeners
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: PATH=/usr/bin:/bin:/usr/sbin:/sbin
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: LOGNAME=macuser
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: XPC_SERVICE_NAME=com.apple.installer.78432
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: USER=macuser
Sep 23 16:30:02 mac-mbp Installer[66897]: Env: XPC_FLAGS=0x0
Sep 23 16:30:02 mac-mbp Installer[66897]: Firebird Database  Installation Log
Sep 23 16:30:02 mac-mbp Installer[66897]: Opened from: /Users/macuser/Downloads/FirebirdCS-2.5.4-26856-x86_64.pkg
Sep 23 16:30:03 mac-mbp Installer[66897]: Top component package /Users/macuser/Downloads/FirebirdCS-2.5.4-26856-x86_64.pkg not found or understood: treat as unsigned
Sep 23 16:30:13 mac-mbp Installer[66897]: LSExceptions [0x7f93d9603070] unloaded
Sep 23 16:30:35 mac-mbp Installer[66897]: InstallerStatusNotifications plugin loaded
Sep 23 16:30:43 mac-mbp Installer[66897]: Administrator authorization granted.
Sep 23 16:30:43 mac-mbp Installer[66897]: ================================================================================
Sep 23 16:30:43 mac-mbp Installer[66897]: User picked Standard Install
Sep 23 16:30:43 mac-mbp Installer[66897]: Choices selected for installation:
Sep 23 16:30:43 mac-mbp Installer[66897]:     Upgrade: "Firebird Database"
Sep 23 16:30:43 mac-mbp Installer[66897]:         FirebirdCS-2.5.4-26856-x86_64.pkg : org.firebirdsql.Firebird : 2.5.0.2.5
Sep 23 16:30:43 mac-mbp Installer[66897]: ================================================================================
Sep 23 16:30:43 mac-mbp Installer[66897]: It took 0.00 seconds to summarize the package selections.
Sep 23 16:30:43 mac-mbp Installer[66897]: -[IFPKGDerivedDocument sortedPackageLocations]: result = (
        "file://localhost"
    )
Sep 23 16:30:43 mac-mbp Installer[66897]: -[IFDInstallController(Private) _buildInstallPlanReturningError:]: location = file://localhost
Sep 23 16:30:43 mac-mbp Installer[66897]: -[IFDInstallController(Private) _buildInstallPlanReturningError:]: file://localhost/Users/macuser/Downloads/FirebirdCS-2.5.4-26856-x86_64.pkg
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.
Sep 23 16:30:43 mac-mbp Installer[66897]: Install failed: The Installer could not install the software because there was no software found to install.
Sep 23 16:30:43 mac-mbp Installer[66897]: IFDInstallController D94322A0 state = 8
Sep 23 16:30:43 mac-mbp Installer[66897]: Displaying 'Install Failed' UI.
Sep 23 16:30:43 mac-mbp Installer[66897]: 'Install Failed' UI displayed message:'The Installer could not install the software.
   
    The Installer could not install the software because there was no software found to install.'.