The existence of a specifically requested local file can be found due to the double firing of the "onerror" when the "source" attribute on a "<track>" tag refers to a file that does not exist if the source page is loaded locally. This vulnerability affects Firefox < 51.
https://www.mozilla.org/security/advisories/mfsa2017-01/
https://bugzilla.mozilla.org/show_bug.cgi?id=1295023