Update:
I tried deleting Clippings from my old
FF56, closed it, and then opened the new FF57, where Clippings was
missing (as expected/intended).
I then tried to re-install
Clippings, but got the message: "Project web is currently offline
pending the final migration of its data to our new datacenter." (just a coincidence?)
I ignored that message and tried to use Clippings in a Gmail mail. The message about restrictive privacy settings has gone and I am able to use the extension again.