Everyone has a story like this - from all of the browsers.
It seems like FF behaves better in this way now, but I still do silly things like rsync my home directory to a backup before every reboot/update. If I ever need these tab backups, I will have to do silly things like dissect them with JSON tools or monkey around with plists, etc.
Why, Why WHY can't we just have tabs:// ?
Plain text list of all URLs. Dead simple. Just pull up tabs:// and cut and paste. Or select all. Or whatever.
Lose-all-tabs bugs will always be with us - and a fix would be so simple ...