When Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-23995
• https://bugzilla.mozilla.org/show_bug.cgi?id=1699835
• https://www.mozilla.org/security/advisories/mfsa2021-14/
• https://www.mozilla.org/security/advisories/mfsa2021-15/
• https://www.mozilla.org/security/advisories/mfsa2021-16/