Developer reply by Janusz-Bliźniak
Rated 1 out of 5 stars
Version 1.2 it is not functional in the version 115.16.2 (64bit). Version 2.0 cannot be installed because it is said to be incompatible with this version. So a stalemate.
Do you have enabled Message Pane (F8) ? Addon saves selected message after displaying its content, so the Message Pane should be visible.
Version 2.0 is for the current version of Thunderbird, which is version 128.
To create your own collections, you must have a Mozilla Add-ons account.