By default Thunderbird sorts the folders of an account alphabetically. This extension allows you to choose a specific sort order for your folders, allowing you to put the most important ones on top, and manually order them.
This extension also offers another method for automatically sorting folders. Unlike Thunderbird's default, the extension's sort method takes into account Unicode special characters. If you do not wish to sort everything manually, you can use Unicode symbols at the beginning of your folder names to put them on top. For instance, you can put a Unicode star ✸ at the beginning of your folder name to put it on top of the list without having to sort everything manually.
This also works for newsgroups inside a news accounts, and also works for sub-folders, and sub-subfolders, etc.
As a bonus feature, you can also re-order accounts as well, there's an extra tab for that now. The latest version now also allows you to choose the default startup folder.
(Note: there have been reports that running the addon through http://addonconverter.fotokraina.com/ makes it compatible with SeaMonkey.)