Hi David,
Reminderfox works with TB 60.3 which is the current official version.
The problem with TB > 60.3 is not with the Reminderfox code itself but -- as with many other addons -- the different changes because of the Mozilla stoppage of it's classic web technology.
The TB development team is working hard to make classic addon work also with TB version up to TB68. For addon's it's required to follow those changes.
The current Reminderfox dev updates work up to TB 63. A further support for beyond TB63 is very much open at the moment.
Guenter