The job of cybersecurity teams is to come up with possible security vulnerabilities and stay on top of them whenever they do arise. Preferably, this job is done in order to catch any issues before malicious actors actually manage to find them and exploit them. Not every vulnerability is caught on time, however, which is how you usually end up with zero-day exploits and vulnerabilities. One that affects Firefox and Thunderbird has just been fixed by Mozilla.
Source : Update Firefox and Thunderbird Now to Fix a Critical Security Flaw