Review for ReminderFox by Sarisisop
Rated 5 out of 5 stars
I've just moved it over to Thunderbird and imported all my reminders and it's working great. Not sure how long it will continue to work in Thunderbird but at least I have my reminders back for now.
Another option is Waterfox it works fine on there, but I'm not sure if that will change.
So sad to see this add on go.
To create your own collections, you must have a Mozilla Add-ons account.