Sorry, the Add-ons team is working on a fix for this. It looks like a certificate used to sign many popular extensions expired. (The verification process checks that certificate.)

I don't know how soon re-signed versions of those extensions will become available, or whether another workaround will be discovered. We are all hoping to learn more soon.

This isn't a problem on your end. This is happening to quite a few people, probably everyone I'd guess. This is only a temporary problem and they are working on fixing it. You are not alone. I'm just glad I found this forum. I was able to get answers, and now I can help other people.