Ohodnotené 4 z 5 hviezdičiek
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... :-(
"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: