When Firefox Quantum (version 57) launched in November 2017, it exclusively supported add-ons built using WebExtensions APIs. addons.mozilla.org (AMO) has followed a parallel development path to Firefox and will soon only support WebExtensions-based add-ons.
As Thunderbird and SeaMonkey do not plan to fully switch to the WebExtensions API in the near future, the Thunderbird Council has agreed to host and manage a new site for Thunderbird and SeaMonkey add-ons. This new site, addons.thunderbird.net, will go live this week.
Starting on July 12th, all add-ons that support Thunderbird and SeaMonkey will be automatically ported to addons.thunderbird.net. The update URLs of these add-ons will be redirected from AMO to the new site and all users will continue to receive automatic updates. Developer accounts will also be ported and developers will be able to log in and manage their listings on the new site.
Thunderbird or SeaMonkey add-ons that also support Firefox or Firefox for Android will remain listed on AMO.
If you come across any issues or need support during the migration, please let us know about it in this thread.