Sorry, sessions aren't saved when you close one window while another window is open. When you have a problem like that you should use Exit to close Firefox completely; that will allow the session to be restored when you re-open Firefox.

Now you'll need to pick thru your browsing history to find those 11 tabs that you closed.

Beyond that, you might want install a session saver extension that would save more than just the last session, along with saving individual windows when you have more than one window open.
AMO search for "session"

Thank you for your prompt reply.

The problem is that a restore-previous-session tab doesn't show up in history and that's the one that really matters to me.

I did back up the sessionstore-files as soon as this happened, can one do some magic with those?

Very smart backing up those files immediately! You may be able to recover the urls for the tabs you're looking for. :)

Please try the recovery steps listed here:
http://forums.mozillazine.org/viewtopic.php?f=38&t=622036&start=60&p=12098147#p12098147

For this to work though you will need to rename your current sessionstore.js file temporarily (sessionstore.old for example) then copy over the backed up sessionstore.js file to your profile. Make sure you do all of this while Firefox is closed.

If that doesn't work, copy your backed up sessionstore.bak file to the Firefox profile and rename it sessionstore.js - usually this does the trick.

If the urls you want are not found there and you had this missing window open in the days before this, you can try the Restore Previous Versions feature of Windows 7 to recover this file. :D There are usually months worth of backups of the Firefox profile folder and you can simply open the backed up folders and copy whichever files you need. I would start with a copy of the sessionstore.js from a Previous Versions folder from before the accident.

Here's a guide explaining how to use Restore Previous Versions:
http://www.howtogeek.com/howto/11130/

To view the folders, navigate to where your Firefox profile folder lives: C:\Users\[Username]\AppData\Roaming\Mozilla\Firefox\Profiles\[random].default\

Right-click on the [random].default folder and choose Restore previous versions. Choose a folder dated from before this accident and click the "Open" button. You should find a safely backed up sessionstore.js file there. ;) Lastly, copy the file to another safe location and copy it back into the Firefox profile.

This is really the best feature Windows ever invented and a big reason to upgrade from XP for those that have lost files.