Review for Duplicate Contact Manager by Gavrahil
Rated 3 out of 5 stars
Very good Add-On. Does what it advertises, but has a few faults nonetheless.
Sometimes you have to manually correct some items, but much more importantly I agree with John Pang: as soon as you're done with the duplicates turn it off, or you'll have no joy with your Thunderbird GUI.
To create your own collections, you must have a Mozilla Add-ons account.