The FBI hacked the Mozilla browser, and the Mozilla Foundation entered a brief in court (PDF here) asking them to disclose how they did it so the vulnerability could be plugged.
The FBI had taken over a child porn website on the dark web/deep web which could then track visitors. The FBI actually found the backdoor in Tor, an anonymous browsing service, which shares code with Mozilla. (I'm assuming the FBI could then get the user's real IP address, which TOR obfuscates.)
Cyberwar News. May 17, 2016.
Archive.is backup of Mozilla PDF brief.