There never was the ability to open Clippings Manager from extension preferences in previous versions of Clippings. That said, it certainly can be done in a WebExtension. I will consider adding this feature if there is significant user interest.
The Mozilla team is rolling out a fix for Firefox which should resolve the certificate issue affecting all add-ons. See their Twitter feed for details: https://twitter.com/mozamo/status/1124627930301255680