How to tell Firebird to only accept conections from XYZ host or network?
This isn't really a thing you should be configuring in Firebird. There is a RemoteBindAddress setting in firebird.conf which configures on which interfaces/addresses the Firebird listens but that's all. You should really use your system's firewall to set this up.
Beside firewall, if you use Classic on Linux, you can use xinetd or inetd access control files /etc/hosts.allow and /etc/hosts.deny. With xinetd you can also edit the xinetd configuration file for Firebird service, which is in /etc/xinetd.d/firebird and add a line like this:
"only_from = 192.168.0.0/24"