Z 4 z 5 hwěžkow pohódnoćeny

wot jak dnja Apryl 30, 2021 ·

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... :-(

Tute pohódnoćenje je za předchadnu wersiju přidatka (2.0.2). 

wot Jorg K (wuwiwar) Apryl 30, 2021 ·

"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