Developer reply by Eyal Rozenberg
Rated 5 out of 5 stars
It was perfect but stopped working in Thunderbird 91. Please fix asap. Thanks.
This review is for a previous version of the add-on (0.11rc1).It didn't "stop working", I just had to update it for TB 91... which has now happened.
To create your own collections, you must have a Mozilla Add-ons account.