Hello everybody, I found the solution of this problem, this is it:

If you pass your FirstForm to the SecondForm class (for example using a constructor parameter) the SecondForm instance can make the original FirstForm instance visible again instead of creating a new one.

For example:

public class SecondForm extends JFrame{
  private final FirstForm firstForm;
  publicSecondForm( FirstForm form ){
  firstForm = form;
 }

private void jButton1ActionPerformed(java.awt.event.ActionEventevt) {                                         
  firstForm.setVisible(true);
  setVisible(false);
  dispose();
 }  
}

and in your FirstForm class:

private void jButton3ActionPerformed(java.awt.event.ActionEventevt) {
 SecondForm secondform = new SecondForm( this );
 secondform.setVisible(true);
 setVisible(false);
 dispose();
}