Developer reply by Itagaki Fumihiko
Rated 3 out of 5 stars
Would be 5* if the Authors explained why they need so high level of trust from the users...
"Required permissions for core functionality:
Have full, unrestricted access to Thunderbird, and your computer"
A blog below explains it clearly.
https://hogiblog.blogspot.com/2020/09/access-permissions-of-add-ons.html
This add-on also uses WebExtension Experiments and will be notified of Case 2 full access requests.
This add-on does not do or need full access. But it uses a conventional mechanism that allows full access. And such add-ons always generate such warnings.
To create your own collections, you must have a Mozilla Add-ons account.