What is the Firebird Guardian?
It is only usable if you run Firebird as application and not as service. For example, that's the only way on Windows 98. It's a small application that checks whether the Firebird server is running and restarts it if it crashes.
If you configure Firebird as service on newer Windows (XP, 2000, etc.) you can set it up to get restarted by Windows, so there's no need for the Guardian (see note about silent install at the end of this text).
On Linux, the Super Server uses guardian as well for the same purpose, the process is called ibguard or fbguard depending on Firebird version and it (re)starts the ibserver/fbserver processes.
Please note that guardian should only be used with SuperServer. When you use Classic each connection is a separate process, so there is not benefit in restarting it as the connection is gone when it dies - and a new one would be spawn by xinetd or inetd anyway.
On Windows, having guardian running can even be counter-productive as it would run fbserver.exe instead of fb_inet_server.exe, which can create problems. Make sure you are not running guardian with classic server.
If you are doing a silent install on Windows, you can disable the Guardian and make Firebird run as service by default using the instsvc.exe tool. For details, please read the README.instsvc in the 'doc' directory of your Firebird installation.