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.
If you don’t wish to be notified about these add-ons anymore, please disable the listing pages on the new site, and the old one, if needed”
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.
CU
Rainer Bielefeld
SeaMonkey Internet Suite 
2018-07-12 at 6:17
The Thunderbird Add-ons site is currently down for extended maintenance.
It should return by July 12 21:00 UTC.
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2018-07-14 at 7:06
The Thunderbird Add-ons site is currently down for extended maintenance.
File copy from the Firefox Add-ons site is taking significantly longer than expected. Due to this, we’ve further extended maintenance until at least July 13, and potentially Monday.
The site will return by July 16 21:00 UTC at the latest.
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2018-07-22 at 17:54
Seems to be done (I was some days on camping holidays), some first tests look promising.
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