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://bugzilla.mozilla.org/show_bug.cgi?id=1674343
https://lists.debian.org/debian-lts-announce/2021/02/msg00001.html
https://lists.debian.org/debian-lts-announce/2021/02/msg00002.html
https://security.gentoo.org/glsa/202102-02
https://www.debian.org/security/2021/dsa-4840