Does that extension create any backups or did you remove those as well?

If you have reset Firefox then a new profile has been created and some of your data (bookmarks, passwords, cookies, form data) is automatically imported and your current profile is moved to the desktop (Old Firefox Data).

You can recover more data from the old profile (be cautious with doing that).

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

As far as I am aware the session manager creates no back ups

• http://sessionmanager.mozdev.org/documentation.html

There is an option to have Session Manager automatically back up your current browsing session at browser shutdown, ask you if you want to save or never save. By default it backs up the ten previous browsing sessions and crashes (deleting older ones). This can be changed in the Startup and Shutdown options on the General Options page. Old sessions are only removed on browser shutdown. Restarting or recovering from a crash will not remove old sessions. Warning, the backup is made when the browser actually shut downs, so if you close the browser by "X"ing out of all the windows, only the last window will be saved. If you want to save the last session, use the File->Exit command or better yet manually save it or use the autosave feature.