Review for RemindIt by darrask
Rated 4 out of 5 stars
I find that very useful. It is better than filtering messages by status because you don't not want to keep track of every unanswered mail. (Especially when you reply "OK" to someone, you would not expect an answer).
This review is for a previous version of the add-on (0.9).To create your own collections, you must have a Mozilla Add-ons account.