You can't do that currently in Thunderbird. Work is underway for Thunderbird to implement this functionality natively, without the need for an add-on. However, this won't ship in the current 115.x release, but only in the next major TB version some time in mid 2024. Alternatively you can give Betterbird a try, the ability to manually sort (order) folders in the folder pane has already been implemented there. https://betterbird.eu/index.html