Further techniques that built on the slipstream research combined with a malicious webpage could have exposed both an internal network's hosts as well as services running on the user's local machine. This vulnerability affects Firefox < 85.
https://www.mozilla.org/security/advisories/mfsa2021-03/
https://www.debian.org/security/2021/dsa-4897
https://www.debian.org/security/2021/dsa-4895
https://security.gentoo.org/glsa/202104-10
https://security.gentoo.org/glsa/202104-09
https://lists.debian.org/debian-lts-announce/2021/04/msg00020.html
https://lists.debian.org/debian-lts-announce/2021/04/msg00019.html