Developer reply by Eyal Rozenberg
Rated 4 out of 5 stars
It takes some tweaking sometimes (i.e. changing up the search criteria) to get all of them, but this add-on is fantastic!! Having recently moved (or more aptly in the process of moving) to Gmail IMAP services, I do see it doesn't work very well in this context (which is really too bad, as I was counting on this extension to help me merge my online and old pop folders and weed out the duplicates)...
David
If you're having trouble with dupe removal on an IMAP account, file a bug using http://www.mozdev.org/bugs or at least email me. The extension does work with IMAP accounts including GMail.
To create your own collections, you must have a Mozilla Add-ons account.