Apologies for the short notice, but Clippings 6 is currently unavailable for download and installation.
Clippings 5.5.2 is still available for Mozilla Thunderbird, and for Firefox ESR 52.x. It can be found in the Past Versions page on the AE Creations website.
A minor release of Clippings 6 is currently in the works and will be released soon. A release announcement will be posted here when it becomes available.