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.

Background: Add-Ons and Add-On Signing

Although many people use Firefox out of the box, Firefox also supports a powerful extension mechanism called “add-ons”. Add-ons allow users to add third party features to Firefox that extend the capabilities we offer by default. Currently there are over 15,000 Firefox add-ons with capabilities ranging from blocking ads to managing hundreds of tabs.

Firefox requires that all add-ons that are installed be digitally signed. This requirement is intended to protect users from malicious add-ons by requiring some minimal standard of review by Mozilla staff. Before we introduced this requirement in 2015, we had serious problems with malicious add-ons.

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