Firefox extensions are signed, and Mozilla allowed one of the certificates to expire, causing lots of inconvenience and even user data loss… As ever, the reasons are not simple - here’s their post mortem:
https://hacks.mozilla.org/2019/07/add-ons-outage-post-mortem-result/
Previously, they posted about the technical background to the mishap:
https://hacks.mozilla.org/2019/05/technical-details-on-the-recent-firefox-add-on-outage/