A vulnerability where WebExtensions can download and attempt to open a file of some non-executable file types. This can be triggered without specific user interaction for the file download and open actions. This could be used to trigger known vulnerabilities in the programs that handle those document types. This vulnerability affects Firefox < 56.
https://www.mozilla.org/security/advisories/mfsa2017-21/
https://bugzilla.mozilla.org/show_bug.cgi?id=1346515