When a "javascript:" URL is drag and dropped by a user into the addressbar, the URL will be processed and executed. This allows for users to be socially engineered to execute an XSS attack on themselves. This vulnerability affects Firefox < 53.
https://www.mozilla.org/security/advisories/mfsa2017-10/
https://bugzilla.mozilla.org/show_bug.cgi?id=1229426