Review for Remove Duplicate Messages by Firefox user a9482b
Rated 5 out of 5 stars
By far the easiest way of getting rid of clutter. It is no longer being updated from the look of it. That is really sad as it one of the most useful plugins available for a mail client. I would happily pay for this plugin if someone was willing to build it as a paid plugin (HINT HINT).
To create your own collections, you must have a Mozilla Add-ons account.