AMO changed the way they serve up addons to ones that expose a full path to the addon (causing ThunderBrowse to recogize the type). Before, the addons would just point to urls that would cause the addon to be downloaded but not cause an install window to appear.
This has been fixed. Hooray!
Also, if you happen to use Personas for Thunderbird, you can apply personas from ThunderBrowse as well.