--Какие-то из этих ссылок лишние)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using NHibernate.Cfg;
using NHibernate;
using FluentNHibernate.Automapping;
using FluentNHibernate.Cfg;
using FluentNHibernate.Cfg.Db;
using FluentNHibernate.Conventions;
using FirebirdSql.Data.FirebirdClient;
using NHibernate.Tool.hbm2ddl;
using System.IO;
using System.Text.RegularExpressions;

 public xxx(String conn_string)
        {
            FirebirdConfiguration fbc = new FirebirdConfiguration();
            Configuration  cfg = Fluently.Configure().Database(fbc.ConnectionString(conn_string))
                .Mappings(m => m.FluentMappings.AddFromAssemblyOf<MyClass>()).BuildConfiguration();

            ISessionFactory  sf = cfg.BuildSessionFactory();
        }