Rated 5 out of 5 stars

Very useful add-on. The only shortcoming for me is that it doesn't work with reply.

This review is for a previous version of the add-on (2.0.5). 

Rated 4 out of 5 stars

Should be integrated in Thunderbird.
If you want to use it, I think the default language of Thunderbird should be English, since the field to modify ("custom 4", in the "other" tab) is in English... :-(

This review is for a previous version of the add-on (2.0.2). 

"Should be integrated into Thunderbird", right, did you read the description of the add-on?
Note that one day in the future Thunderbird will absorb this add-on, since there is an enhancement request for it:
https://bugzilla.mozilla.org/show_bug.cgi?id=1020181