Rated 4 out of 5 stars
Does not do anything in THunderbird 15.0.1. Messages still are not threaded and makes no difference if the extension is enabled or not with all the default options enabled.
Seems the problem is related to the mailing list munging message headers. Makes it almost impossible to follow threads properly.
There's a FAQ over there, see items "messages are not grouped together". https://github.com/protz/GMail-Conversation-View/wiki/faq
Also make sure there are no errors in Tools > Error console.