Rated 5 out of 5 stars

This addon adds Gmail's "killer feature" to Thunderbird: flat threading in a single view. The developer is very active fixing bug reports and even feature requests, ensuring compatibility with other extensions, and ensuring that the extension works in all usage scenarios. Issues mentioned in other reviews, such as the attachments issue (Pierre) are being tracked in the GCV bugtracker already:
http://github.com/protz/GMail-Conversation-View/issues#issue/39

For those unfamiliar with Gmail's threading, this extension presents conversations as a list of all messages, both the contact's messages and your own. For those familiar with Gmail, like the microwave, we cannot function without it anymore!

This review is for a previous version of the add-on (0.3beta2).