Comment 17 for bug 1901829

And this (French) forum post has more insights: https://forum.ubuntu-fr.org/viewtopic.php?pid=22359410.

I can reliably reproduce the problem by doing the following (on my system where the locale is fr):

    sudo apt remove thunderbird-locale-fr
    find ~/.thunderbird -name <email address hidden> -exec rm {} \;
    # launch thunderbird (now in English), open the Add-ons Manager, search for "Français" and install the first result ("Français Language Pack (Thunderbird)")
    # close thunderbird, and open it again, and observe the popup with the XML parsing error