A “zero-day exploit” is a term used for critical vulnerabilities that is found for the first time, and it is crucial for teams to act quick and release patches. It is equally crucial for browser users to download the patch and update their browsers. Firefox has rated this exploit in its highest category: “Critical Impact — Vulnerability can be used to run attacker code and install software, requiring no user interaction beyond normal browsing.”
Earlier today the Mozilla team released a patch in Firefox version 67.0.3. Again, it is critical that all Firefox users, whether cryptocurrency users or not, update their browsers as soon as possible.
(Like now, yes you.)
edit: Some more information about the vulnerability has been provided by the Center for Internet Security.