Zombie Keys (Multilanguage Keyboard) 2.15
Enter accents, diacritics, diaeresis, umlauts, ligatures etc. with keyboards of various countries (us,uk,ie,fr,it,ru,de,sv) - via easy to remember shortcuts or menu. Also supports all textboxes of the application (such as search, filters).
This add-on is not compatible with your version of Thunderbird because of the following: