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If a site is loaded over a HTTPS connection but loads a favicon resource over HTTP, the mixed content warning is not displayed to users. This vulnerability affects Firefox < 63. Reference: https://bugzilla.mozilla.org/show_bug.cgi?id=1484753
External References: https://www.mozilla.org/en-US/security/advisories/mfsa2018-26/#CVE-2018-12403
Red Hat Enterprise Linux 7 is running 60.6.0, which is presumably impacted.
Reproduced this on the firefox version shipping with Red Hat Enterprise Linux 7. 5 and 6 were not tested, but I am guessing that they are vulnerable.