using FirebirdSql.Data.FirebirdClient;
FbConnectionStringBuilder fb_con = new FbConnectionStringBuilder();
fb_con.Charset = FbCharset.None.ToString();
fb_con.UserID = "SYSDBA";
fb_con.Password = "masterkey";
fb_con.Database = "mainDB.FDB";
fb_con.ServerType = FbServerType.Embedded;
fb = new FbConnection(fb_con.ToString());
//создаем подключение
//string connectionString = string.Format("User={0};Password={1};Database={2};DataSource=localhost;Charset=NONE;ServerType=1;ctype_none=NONE;", "", "", @"");
fb = new FbConnection(fb_con.ToString());
fb.Open();
FbDatabaseInfo fb_inf = new FbDatabaseInfo(fb);
FbCommand selCom = new FbCommand("select CAT0_NAME from CAT0", fb);
FbDataReader dr = selCom.ExecuteReader();
while (dr.Read()) {
    string inS = dr[0].ToString();
    var newStr = Encoding.ASCII;
    byte[] getBytes = newStr.GetBytes(inS);
    inS = utf8.GetString(getBytes, 0, getBytes.Length);
    textBox1.AppendText(inS + "\n");
}
dr.Close();

select * from rdb$database