Mozilla Thunderbird: Creating Folders

I don't remember if this was a previous post or not, but...

just get to your folder (see Navigating Folders)

this will be an nsIMsgFolder.

Use nsIMsgFolder.createSubfolder(folder_name,msgWindow);
As may be inferred, this will create the subfolder, but you have to update the parent folder to get it to show up in the Folder Pane.

parentFolder.updateFolder(msgWindow);

If you want to learn more about nsIMsgFolder, go to the XulPlanet article.