I assume you mean the user on the database server is sysdba (it's the default Firebird admin user).
A user on the Firebird server is a different concept than an operating system user.
The Firebird server software itself is started under a certain user account as well (could be root, or indiana, or perhaps a new user firebird set up by the installer).
When Firebird is running, a command like ps aux | grep -i firebird should show under which operating system user Firebird is running.