If you are using the regular version of Firefox, you need to upgrade from 74.0 to 74.0.1 and if you are using Extended Support Release (ESR), you will need to upgrade from version ESR 68.6.0 to ESR 68.6.1.
Zero-day vulnerability could be attributed to the Firefox 68 version, which was released in July 2019. It could, however, be a "side effect" of a patch of the 68.0 version.
(If you have Firefox ESR version XY0, stick to the Firefox X.0 feature set, but with all the updates security which have even come out of Firefox (X + Y) .0).
However, there are no details on when the Firefox error was discovered by them hackers and how exactly they take advantage of it.
Right now, Mozilla says only that:
What does use-after-free mean?
The use-after-free is a category of errors caused by improper use memory blocks from one program.
Usually, a program "returns the blocks of memory" to the operating system system after finishing with them, allowing the memory to be used again for something else.
The function by which the μνήμη is returned for reuse, called free (), and once you release the memory, you obviously can't access it again.
In this process some mistake can be made and enter code in data which were made in such a way by a swindler to deceive you.
Not all use-after-free errors are exploitable and also cannot all cause the same problem. For example, an intruder can only change the contents of an icon or message you want to show off, which could be used to deceive others users.
However, in some cases, errors can allow an intruder to change the flow of control within the program. It could for example affect the CPU to execute unreliable code, that the intruder "threw" in memory, bypassing his security checks browser.
This is the most serious kind of vulnerability. It is also known as RCE and refers to remote code execution. The criminal can execute the code on your computer even if it is on the other side of the world.
What can we do;
If already some hackers managed to take advantage of it vulnerability, others will. Therefore, the most common but effective treatment is regular updating of systems.
Most Firefox users should automatically receive updates, but it's a good idea to check to make sure.
Click the menu (three-line icon) in the upper-right corner, and then select Help> About Firefox.