hello please make sure, that there's no external program on your pc clearing up firefox' history data. some security programs, clean-up-& tuning software and even programs like hp support assistant that ships with hp hardware come with this functionality...

If Firefox has crashed or otherwise hadn't closed properly then session data may not have been saved or files may have become corrupted.

You can check the settings for Clear Recent History.

• https://support.mozilla.org/kb/Clear+Recent+History

You can try to do some maintenance on the places.sqlite database file:

• Places Maintenance: https://addons.mozilla.org/firefox/addon/places-maintenance/