Review for Thunderbird Conversations by Shion3072
Rated 3 out of 5 stars
this wasn't for me, with a lot of different accounts already the way this sorted things out wasn't much to my liking. So I was quite pleased that on uninstall everything went back exactly as it was except marking a lot of read emails as unread.
Which means, give it a try,
To create your own collections, you must have a Mozilla Add-ons account.