Rated 2 out of 5 stars

Sorry Mate. It's not working at all with TB 60.4.0. Which is actually NOT all your fault, since many other Addons are not usable as well after that last (unutterably) update. That's a shame.
Seems like the TB developers are incapable to include a functioning duplicate remover themself. Instaed they creating "pretty" little buttons. I am fed up with that constant updates and stopped the TB updates from now on as I think its an unreasonable demand AND a mess.

I know it may not work with TB 60.4 - I haven't attempted to get it to work with that version. As you've gathered, Thunderbird is undergoing massive changes under the hood, and currently it is a fast-moving target. Any changes I make to my extension to accommodate a certain version may (or may not) be invalidated in a couple of months. Also, the process of adaptation is not well-documented, if at all, unlike in previous versions.

If I had a lot of spare time, I'd probably do it anyway, but right now I just can't. Do check again in a few months.

For now, sadly, I can only recommend you downgrade your Thunderbird version if you want to use the extension, then upgrade back.

My apologies,
Eyal