Rated 5 out of 5 stars
This STILL works, even up to version 11.0 of Thunderbird, but you will need to edit the maxversion to read 11.* in the install.rdf file which is inside the XPI (addon) file. It can be opened as a .ZIP file.
The addon is quite nice, and immediately gives you an address sidebar and an addons sidebar without having to add anything additional.