A legacy extension's non-contentaccessible, defined resources can be loaded by an arbitrary web page through script. This script does this by using a maliciously crafted path string to reference the resources. Note: this vulnerability does not affect WebExtensions. 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=1432870