How does the browser restores the previous session if the session is closed accidentally?

How does the browser restores the previous session if the session is closed accidentally? For example, If I have a page open in Firefox, and log off, next time I open Firefox, it asks me whether I want to restore the previous session or start a new one. How does this work?|||It probably saves all the open websites to an XML file and it reads this to determine what pages to open next time Firefox starts.|||it holds it is "cache" which is your temporary internet files....|||Cookies, cached memory