Firefox requires extensions installed via their "store" be signed with a certificate to make sure they're actually from there. That certificate has an expiry date. It expired, so now all of its signatures are invalid -- and Firefox no longer trusts the associated extensions.
A minor added detail is that it wasn't the leaf certificate that expired. I've heard that that would have been handled properly. It was an intermediate cert, and I guess that possibly wasn't fully taken into account? (This is 3rd hand knowledge and speculation, note.)