Review for Duplicate Contact Manager by David von Oheimb
Rated 3 out of 5 stars
In response to recent requests/reviews:
The original developer has abandoned this add-on and has been unreachable for years. Since I needed some improvements I ended up doing them myself and providing a strongly improved add-on here: https://addons.thunderbird.net/en-US/thunderbird/addon/duplicate-contacts-manager/
To create your own collections, you must have a Mozilla Add-ons account.