Technical details on the recent Firefox add-on outage

Recently, Firefox had an incident in which most add-ons stopped working. This was due to an error on our end: we let one of the certificates used to sign add-ons expire which had the effect of disabling the vast majority of add-ons. Now that we've fixed the problem for most users and most people's add-ons are restored, I wanted to walk through the details of what happened, why, and how we repaired it. An in-depth look at the cause and fixes for the devastating extensions bug that hit Firefox users over the weekend, written by Firefox CTO Eric Rescorla.