Earlier quoted context omitted.
It's not XUL anymore, actually, as far as I know. That's why it ripped out support for XUL addons.
Firefox has been ripping out xul, but Thunderbird still appears to support it.
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Add-on support: Add-ons are only supported if add-on authors have adapted them
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Dictionary support: Only WebExtension dictionaries are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension dictionaries.
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Theme support: Only WebExtension themes are supported now. Both addons.mozilla.org and addons.thunderbird.net now provide WebExtension themes.
https://www.thunderbird.net/en-US/thunderbird/68.0/releaseno...