firefox is vulnerable to information disclosure via spoofable website .A malicious extension could have opened a popup window lacking an address bar. The title of the popup lacking an address bar should not be fully controllable, but in this situation was. This could have been used to spoof a website and attempt to trick the user into providing credentials.
bugzilla.mozilla.org/show_bug.cgi?id=1693664
security-tracker.debian.org/tracker/CVE-2021-23984
security.gentoo.org/glsa/202104-09
security.gentoo.org/glsa/202104-10
www.mozilla.org/security/advisories/mfsa2021-10/
www.mozilla.org/security/advisories/mfsa2021-11/
www.mozilla.org/security/advisories/mfsa2021-12/