Since th last Update of Thunderbird to 68.3.1 Reminderfox doesn't work correct.
Hi Walter!
Due to significant changes in Thunderbird's support for add-ons,
ReminderFox will not work in version 68 and above. See
https://groups.google.com/d/msg/reminderfox/MlYIeFOtpMs/0RiFOQbuAwAJ
for more details. Note that at that time it was expected that the
next major Thunderbird version would have the version number 65.
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
Let us know if you need more information.
Paul