If the Compact() method was called on an nsTArray, the array could have been reallocated without updating other pointers, leading to a potential use-after-free and exploitable crash. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
https://www.mozilla.org/security/advisories/mfsa2020-52/
https://www.mozilla.org/security/advisories/mfsa2020-51/