What is Reddit's opinion of ReminderFox?
From 3.5 billion Reddit comments

➔ ReminderFox website

By popularity on Reddit, this Service is:

#113 in Todo List apps

1 review of this app found across Reddit:

Legacy

In the Classic Add-ons Archive:

caa:addon/reminderfox

2.1.6.3 works, with the toolbar in Waterfox Classic 56.2.13 (pictured here with Vertical Toolbar):

https://s.put.re/yPXMDxrh.png

Nit: the post-installation link to a http://www.reminderfox.org/ URL is outdated.

Later versions

https://addons.thunderbird.net/thunderbird/addon/reminderfox/versions/

2.1.6.6 seems unable to present its toolbar icon with Waterfox Classic 56.2.13.