Apologize problems with the updating process.
Also a new Reminderfox version 2.1.6.6 has been accepted by Mozilla the indexing of the addon is somewhat confusing.
Because of a bug with the Addon Manager pages Reminderfox is noted as "Compatible with Firefox 57+" This NOT the case. Another effect of that bug is also there are some misleading links directing to Firefox addon pages.
Please ignore those points.
Reminderfox 2.1.6.6 -- which is compatible with Thunderbird up to TB60.4 (and Waterfox) -- can be found with this link:
https://addons.thunderbird.net/en-us/firefox/addon/reminderfox/
Initiate direct downloading the latest version with this link:
https://addons.thunderbird.net/thunderbird/downloads/latest/reminderfox/addon-1191-latest.xpi?src=search
In the hope Mozilla gets the things resolved.
Enjoy Reminderfox
Günter / Reminderfox Team