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