The add-on installs in TB 68 but has a number of issues with tags and dates. Read the reviews here: ReminderFox :: Add-ons for Thunderbird
mailmindr is a similar add-on that has its own issues, but is still being developed for 68:
ReminderFox data can be imported to Lightning:
https://groups.google.com/d/msg/reminderfox/ru85YsPd4Qc/xlcdKtfvAQAJ