That shortcut is used by another program on my computer but instead brings up a "page not found" message in Firefox
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Shift+Ctrl+N is the command to restore the last closed window from History > Recently Closed Windows.
There is an addon called Customizable Shortcuts you could use to change this shortcut. Please see the following article.
Let us know if this solves your issue.
There are other things that need your attention.
Please Update to the latest Firefox 20.0.x version.
- Help > About Firefox
The Firefox Firefox 8.0.1 version that you currently run is no longer supported with security updates