[Someone1234]

In the security bug fixes:

>>> CVE-2020-12387: Use-after-free during worker shutdown

>>> Impact: critical

>>> A race condition when running shutdown code for Web Worker led to a use-after-free vulnerability. This resulted in a potentially exploitable crash.

And:

>>> CVE-2020-12388: Sandbox escape with improperly guarded Access Tokens

>>> Impact: critical

>>> The Firefox content processes did not sufficiently lockdown access control which could result in a sandbox escape. [On Windows]

So a sandbox escape and a way to be in a position to need a sandbox escape.

[cesarb]

> CVE-2020-12388: Sandbox escape with improperly guarded Access Tokens

Perhaps it's the same as the Chromium one reported here recently? https://news.ycombinator.com/item?id=22945630