Review for Thunderbird Conversations by throwawaylogin
Rated 4 out of 5 stars
Thunderbird has something similar now, but yours is nicer. 5+ stars if you add a function to drag messages into and out of a thread. (Threads are often incorrect because people don't reply properly.) Outlook has had that forever, it's the only thing I miss in Thunderbird.
This review is for a previous version of the add-on (0.2.1).To create your own collections, you must have a Mozilla Add-ons account.