Also make sure to save a backup copy of the sessionstore.bak file. When Firefox is started then Fire

Also make sure to save a backup copy of the sessionstore.bak file.

When Firefox is started then Firefox renames the sessionstore.js file to sessionstore.bak to make it possible to restore the previous session at any time during the current session (History > Restore Previous Session) and creates a new sessionstore.js file for the current session.

I'm not sure if you would be able to restore a previous version on the sessionstore.js file on Windows 8.1.

Right-click: Properties > Previous Versions

http://windows.microsoft.com/en-us/windows-8/how-use-file-history