A race condition when running shutdown code for Web Worker led to a use-after-free vulnerability. This resulted in a potentially exploitable crash. This vulnerability affects Firefox ESR < 68.8, Firefox < 76, and Thunderbird < 68.8.0.
https://www.mozilla.org/security/advisories/mfsa2020-18/
https://www.mozilla.org/security/advisories/mfsa2020-17/
https://www.mozilla.org/security/advisories/mfsa2020-16/
https://usn.ubuntu.com/4373-1/
https://security.gentoo.org/glsa/202005-04