Problem seem to be in file /usr/lib/thunderbird/omni.ja which is an archive file.
Inside it chrome/messenger/content/messenger/messenger.xhtml has in line 905 a reference to an undefined XML entity &lightning.keys.event.showCalendar.key;
Problem seem to be in file /usr/lib/ thunderbird/ omni.ja which is an archive file.
Inside it chrome/ messenger/ content/ messenger/ messenger. xhtml has in line 905 a reference to an undefined XML entity &lightning. keys.event. showCalendar. key;