When a HTTPS pages was embedded in a HTTP page, and there was a service worker registered for the former, the service worker could have intercepted the request for the secure page despite the iframe not being a secure context due to the (insecure) framing. This vulnerability affects Firefox < 84.
https://www.mozilla.org/security/advisories/mfsa2020-54/
https://www.debian.org/security/2021/dsa-4842
https://www.debian.org/security/2021/dsa-4840
https://security.gentoo.org/glsa/202102-02
https://lists.debian.org/debian-lts-announce/2021/02/msg00002.html
https://lists.debian.org/debian-lts-announce/2021/02/msg00001.html