Find Control in Page of Master Page

   

 ContentPlaceHolder mpContentPlaceHolder;
        TextBox mpTextBox;
// MainContent is content place holder name
        mpContentPlaceHolder =
            (ContentPlaceHolder)Master.FindControl("MainContent");
        if (mpContentPlaceHolder != null)
        {
            string str = "txtFirstName";
            TextBox txt = (TextBox)this.Master.FindControl(str);
            Response.Write(txtFirstName.Text);
        }