I had been using it for a long time without issues, it saved me 2-3 times; but yesterday my computer crashed. My waterfox session file must have been corrupted and it restored it from the recovery one (previous.json) that is somehow 2 months old (thank you mozilla for not updating it more frequently). I immediately went to tab session manager to restore; but surprise surprise, all the backups are gone. At some point it seems the automatic backup feature stopped working and somehow even the older sessions disappeared. Idk if it was a browser update, error or that they were older than X and automatically deleted.
TLDR, even having this extension enabled to prevent the loss of my session, I still lost my session. I will either look for an alternative extension and/or start backuping the session file manually.