Just noting for forum that the latest Mozilla Thunderbird mail client, version 60.3.3, disables older addons, like X-Notifier 3.5.23.
Perhaps someday, the Thunderbird group will update it enough that XN 4.x could be used.
See... https://blog.mozilla.org/addons/2018/07/17/the-new-thunderbird-add-ons-…