I have cracked the 'hidden files' and found where the ReminderFox data is stored (with various backups) - but I need a browser with ReminderFox in order to restore the 'diary' and for that I will need a version of Firefox earlier than 'Quantum'.
I'm guessing that if I uninstall the latest version of Firefox then download and install an earlier version (if I can find one) then install ReminderFox . . .