What OS? What version of Firefox? Yes it happens, but here a few links that should help restore your session before the issue as long as the '''sessionstore-backups folder''' contains files dated before the issue. See the screenshot of my folder and the dates I have. This is why I make a copy of the folder when I remember to do so. https://support.mozilla.org/en-US/questions/1449521 https://support.mozilla.org/en-US/questions/1440459#answer-1637875