Hi Michael!
In addition to Guenter's reply below, if you want to keep Thunderbird v68.3.1, you can easily move from ReminderFox to the Lightning calendar that is included with Thunderbird by importing your ReminderFox .ics file to Lightning. There's information on the Lightning Calendar at https://support.mozilla.org/en-US/products/thunderbird/calendar
I've found that the Lightning calendar works very well.
Paul