One possible solution for the “Many old type Firefox addons lost!” — problem (all FF-CLASSIC-addons dropped from AMO without having moved them to new repository) might be the:
Classic Add-ons Archive
After some trouble I managed to install it to SeaMonkey, but I doubt that it works. I observed 10 minutes of database provisioning … without visible progress. I hope it will be possible to fix that.
I will keep you up to date here.
SeaMonkey Internet Suite 
2019-01-06 at 17:39
With some more careful reading I would have saved much work and time.
After having installed the correct ca-archive-1.2.2.xpi from the13 releases – section add-on now seems to work fine.
I will report experience with hosted add-ons soon, here in the blog.
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2019-01-06 at 18:44
Success!
I managed to Install a working ReminderFox 2.1.6.3 to a new User Profile from the Classic Add-Ons Archive. That one also is available from ATN — I wanted a quick result with an add-on I know.
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