OK, so there has been a lot of grieving for the demise of ReminderFox[RF], me also. I switched to WaterFox over a year ago and moved everything. It was 'bloated' and froze quite often, so I've relentlessly search for alternative browser to keep RF(5yr data). Found one by RF doesn't work in it. Kept WaterFox running with no tabs open, just for RF. One day accidentally closed Waterfox, RF stayed open, fully functional, works like a standalone program. So now ReminderFox can live on!
Hi!
Have you checked Task Manager, or the equivalent for your operating system? Based on my experience (ReminderFox as an add-on in Thunderbird in Windows), when I've accidentally closed Thunderbird before closing all the ReminderFox windows, you'll find that WaterFox is still running, even though its window is closed.
Further, there is no way to start ReminderFox without first
starting the underlying WaterFox/Thunderbird etc., so once the
underlying programme doesn't support ReminderFox, you won't be
able to start ReminderFox
Since you're okay running a programme just for the purpose of running a calendar, and as it even looks like Thunderbird won't support ReminderFox soon, I'd suggest that you investigate the Lightning calendar in Thunderbird - see https://support.mozilla.org/en-US/products/thunderbird/calendar You'll be able to import your current reminderfox.ics file - see https://support.mozilla.org/en-US/questions/1105153 - and so you will be able to maintain all your existing data in a system that is currently actively maintained.
Paul