Not a good few days for Mozilla and Firefox. They just pushed 67.0.3[0] two days ago to fix a zero day.
[0] https://www.mozilla.org/en-US/security/advisories/mfsa2019-1...
[0] https://www.mozilla.org/en-US/security/advisories/mfsa2019-1...
The more code inspected, the more zero days that are identified. Every large code base has security issues. What is scary are the ones that only the wrong people know about.