Critical Vulnerability can be used to run attacker code and install software, requiring no user interaction beyond normal browsing.
Ensure you are using the latest Firefox build.

Security Vulnerabilities fixed in Firefox 72.0.1 and Firefox ESR 68.4.1

...

Incorrect alias information in IonMonkey JIT compiler for setting array elements could lead to a type confusion. We are aware of targeted attacks in the wild abusing this flaw.
Security Advisories for Firefox