How do I change settings on firefox so the page doesn't save when you close the window?

My dad was using my computer and when you close firefox, this little box shows up asking if you want to save and close and a little check box that also says save settings. My dad clicked that box. How do I undo it because its really annoying and my private information can get out. Please help!!!!!How do I change settings on firefox so the page doesn't save when you close the window?
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