About Thunderbird 115+ support: The Thunderbird internal implementation of the message list and what ability there will be for add-on maintainers to customize it are still in flux. A decision about porting this add-on to TB115+ will be made once they settle down.
AMO home page, from which the add-on can be downloaded and installed: https://addons.mozilla.org/thunderbird/addon/enhanced-priority-display/
Author: Jonathan Kamens <jik+epd@kamens.us>
Copyright (c) 2012,2013,2015 Jonathan Kamens. Copyright (c) 2021,2022 Extended Thunder Inc.
Released under the terms of the Mozilla Public License, v. 2.0. The MPL can be found in the file LICENSE.