By exploiting an Open Redirect vulnerability on a website, an attacker could have spoofed the site displayed in the download file dialog to show the original site (the one suffering from the open redirect) rather than the site the file was actually downloaded from. This vulnerability affects Firefox < 81, Thunderbird < 78.3, and Firefox ESR < 78.3.
https://www.mozilla.org/security/advisories/mfsa2020-44/
https://www.mozilla.org/security/advisories/mfsa2020-43/
https://www.mozilla.org/security/advisories/mfsa2020-42/
https://www.debian.org/security/2020/dsa-4770
https://security.gentoo.org/glsa/202010-02
https://lists.debian.org/debian-lts-announce/2020/10/msg00020.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1641487
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00077.html
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00074.html