Review for ThreadVis by jeehan
Rated 5 out of 5 stars
i use this addon for years now and didn't want to miss it.
it is extremely useful if you have a large folder structure and use "copy sent to current" addon too. with ThreadVis it is always possible to retrace a mail thread.
only drawback i found: i found no way to disable ThreadVis on "Sent" folder
To create your own collections, you must have a Mozilla Add-ons account.