Relevant context:

https://bugzilla.mozilla.org/show_bug.cgi?id=1548973#c57

> Update: We have rolled out a partial fix for this issue. We generated a new intermediate certificate with the same name/key but an updated validity window and pushed it out to users via Normandy (this should be most users). Users who have Normandy on should see their add-ons start working over the next few hours. We are continuing to work on packaging up the new certificate for users who have Normandy disabled.

Further context: https://news.ycombinator.com/item?id=19823701