A mechanism to inject static HTML into the RSS reader preview page due to a failure to escape characters sent as URL parameters for a feed's "TITLE" element. This vulnerability allows for spoofing but no scripted content can be run. This vulnerability affects Firefox < 53.
https://www.mozilla.org/security/advisories/mfsa2017-10/
https://bugzilla.mozilla.org/show_bug.cgi?id=1321247