Really useful, I use it every day. I'm also very happy because I couldn't imagine that this can be rewritten as a WebExtension. The Clippings Manager UI looks way worse than in the legacy version, but that's WebExtension. :( The only thing I miss is a link from the options page to the Clippings Manager. It can be reached from the context menu and the toolbar icon, but it would be logical to link it from the options. (I had the Clippings icon in the menu where I could access it when I needed once a year. Until now, because extension icons can no longer be put in the main menu, only in another, which means I can't save an icon place with moving Clippings in that menu. So I decided to remove the icon completely from the toolbar.)
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