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 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.