No longer maintained? Rated 3 out of 5 stars

The idea is simply doubling up the functionality of thew folder plane one the left - I am not quite sure whether this is too useful, as it halves the scrollable area - I guess you can set it up similar to outlook where you can have a "favorite" panel on top of the folders. One problem with this approach that if you need shortcuts because of an overly populated folder tree, you actually worsen the problem by taking away valuable vertical scroll space.
Another problem is that both folder panels seem to compete for focus so you need to look twice in order to know which folder you are currently in. I have had a brief look at the source code and saw that it(temporarily) overwrites some global helper method GetFolderTree - this code is not thread safe, which means that it can break other extensions or the behavior of Thunderbird itself.

onSelect: function(aEvent) {
var tmp = GetFolderTree;
GetFolderTree = gAFViewOverlay.additionalFolderTree.ref;
FolderPaneSelectionChange(aEvent);
GetFolderTree = tmp;
},

instead of doing this, the author should consider copying the functionality of the FolderPaneSelectionChange function so that it is used for his own Folder Pane.

If you are using the additional folder tree mainly for shortcuts, I would also suggest to try QuickFolders. It does a better job for laying out quick dragable folder locations in a space saving and practical manner - as it creates a multi dimensional / tabbed area similar to Tab Mix Plus and the Personal Bookmark Toolbar.