main/network/mail/thunderbird/files/vendor.js

// Use LANG environment variable to choose locale
pref("intl.locale.matchOS", true);

// Disable default mailer checking.
pref("mail.shell.checkDefaultMail", false);

// Don't disable our bundled extensions in the application directory
pref("extensions.autoDisableScopes", 11);
pref("extensions.shownSelectionUI", true);