Mozilla's add-ons SDK had a world-accessible resource with an HTML injection vulnerability. If an additional vulnerability allowed this resource to be loaded as a document it could allow injecting content and script into an add-on's context. This vulnerability affects Firefox < 50.1.
https://www.mozilla.org/security/advisories/mfsa2016-94/
https://bugzilla.mozilla.org/show_bug.cgi?id=1315435