When trying to load a cross-origin resource in an audio/video context a decoding error may have resulted, and the content of that error may have revealed information about the resource. This vulnerability affects Firefox < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.
https://www.mozilla.org/security/advisories/mfsa2021-09/
https://www.mozilla.org/security/advisories/mfsa2021-08/
https://www.mozilla.org/security/advisories/mfsa2021-07/
https://www.debian.org/security/2021/dsa-4866
https://security.gentoo.org/glsa/202104-10
https://security.gentoo.org/glsa/202104-09
https://lists.debian.org/debian-lts-announce/2021/03/msg00000.html