Image for moz-icons can be accessed through the "moz-icon:" protocol through script in web content even when otherwise prohibited. This could allow for information leakage of which applications are associated with specific MIME types by a malicious page. This vulnerability affects Firefox < 59.
https://www.mozilla.org/security/advisories/mfsa2018-06/
https://usn.ubuntu.com/3596-1/
https://bugzilla.mozilla.org/show_bug.cgi?id=1424261