Public Class Form1
Dim firstForm As Form1
You're trying to declare an object of the class within the definition of the class itself.
Also, secondForm would belong to firstForm, so, IIRC, if you close firstForm, you'd also lose secondForm.
Way out: In the module, form, or class that creates the instances of Form1 and Form2, put in an event handler for the button click on the Form1 instance and deal with the showing and hiding/closing there.