I received an e-mail from “The Thunderbird & Add-ons teams”:
” … Starting on July 12th, SeaMonkey add-ons will be listed on addons.thunderbird.net, which is the new home for Thunderbird and SeaMonkey add-ons. This new site is being run by the Thunderbird Council. If the add-ons also support Firefox or Firefox for Android, they will remain listed on AMO. Otherwise, their listings will be removed, and their URLs will redirect to the new site. Similarly, update URLs will be redirected, so all of your users will continue to receive automatic updates on the new site if you upload new versions to it. Your developer account will also be ported, so you should be able to log in and manage your listing.
If you come across any issues or need any support during this transition, please leave a comment on this Discourse thread.
I tested the new site, currently (as expected) still “503 Service Temporarily Unavailable”
May be we will have to adapt SeaMonkey Help, website, wiki, …. All that should be handled via Bugzilla as soon as the new site will be running.