Theme and userChrome for Thunderbird, for use with Thunderbird 102.9.1 or lower.
- In Settings > General > Config Editor... set toolkit.legacyUserProfileCustomizations.stylesheets to TRUE
- Download the whole project.
- Zip both files in the extensions folder into a zip archive, then change the archive extension from .zip to .xpi.
- Drag-and-drop the .xpi into the themes tab of Thunderbird to install.
- Copy chrome/userChrome.css into your Firefox chrome profile folder.
Change the layout with Menu > View > Layout > Vertical View.