Editor’s Note: May 9, 8:22 pt – Updated as follows: (1) Fixed verb tense (2) Clarified the situation with downstream distros. For more detail, see Bug 1549886.

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.