[lmikle]

1. Не ShowModal, а просто Show;
2. Как-то так:

Код:

uses Unit2; // Form2 

procedure Form1.FormCreate(Sender : TObject);
begin
  // To be sure that Form2 is not assigned
  Form2 := Nil;
end;

procedure Form1.Button1Click(Sender : TObject);
begin
  Form2 := TForm2.Create(Application);
  Form2.Show;
  Form2.Update;
  WebBrawser1.Navigate(...);
end;

procedure Form1.WebBrowser1DocumentComplete(Sender : TObject);
begin
  If Assigned(Form2) Then
    FreeAndNil(Form2);
end;