Mozilla filed a motion with the U.S. District Court in Tacoma, Wa., asking the government to disclose a vulnerability it exploited in the Tor Browser and Firefox. The FBI used the zero-day to hack a child pornography site and de-anonymize users visiting the site. The Tor Browser is partially built on open source Firefox code, but also includes proxy code that encrypts and anonymizes users sessions as they move about the Internet. Mozilla argues that the government had previously exploited the Tor browser and that it did so in this case.
Source: https://threatpost.com/motion-filed-asking-fbi-to-disclose-tor-browser-zero-day/118043/