Mozilla removed the option "Don't load tabs until selected" from Firefox 47 user interface.
If you want to load all tabs immediately on Restore, go to about:config and set the preference browser.sessionstore.restore_on_demand to false.
Sessions Manager backup
Sessions are stored in the session.rdf file in your profile folder. A backup of this file is stored in the sessionbackups folder in your profile folder. If your session becomes corrupt and you are unable to restore that session, copy one of the backup files (tabmix_sessions-yyyy-mm-dd.rdf) from the backup folder to your profile folder, rename the file to session.rdf.
Sessions Manager extension
For a full range of sessions managing features, you can use
Session Manager extension which is created specifically for that
When Session Manager extension is installed, Tab Mix Plus turns its own session manager off. Your existing sessions will be converted to SM format. This two extensions work well together without any known problems.
IMPORTANT! If you uninstall or disable Session Manager extension, Tab Mix Plus will revert back to its file which will not have the sessions saved with Session Manager extension.
Read more in our Session Manager help page