It's not so much of an upgrade: Clippings had to be rewritten to be compatible with Firefox 57 and newer, which imposes new requirements for Firefox add-ons. Some features like the shortcut key list and colored labels didn't make it into Clippings 6, but will be considered for upcoming releases. I will continue to monitor feedback from other users before deciding on what to do with the search functionality in Clippings Manager (I assume this is the "search tool" you were talking about).
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