ATN home page, from which the add-on can be downloaded and installed: https://addons.thunderbird.net/addon/folder-pane-view-switcher/
Author: Jonathan Kamens <jik+fpvs@kamens.us>, Klaus Bücher/opto
Copyright (c) 2011,2012,2015 Jonathan Kamens. Copyright (c) 2021,2022 Klaus Bücher/opto.
Released under the terms of the Mozilla Public License, v. 2.0. The MPL can be found in the file LICENSE.