This is about the Freedom Hosting hack in 2013. In 2013 Wired wrote

> On August 4, all the sites hosted by Freedom Hosting — some with no connection to child porn — began serving an error message with hidden code embedded in the page. Security researchers dissected the code and found it exploited a security hole in Firefox to identify users of the Tor Browser Bundle [https://www.wired.com/2013/09/freedom-hosting-fbi/]

However, as far as we know, unlike the more recent Playpen thing, in the Freedom hosting case the FBI did not actually serve child pornography, they just displayed an error message. I don't see anything in this article that suggests otherwise.