Comment 6 for bug 1423441

I am able to reproduce this issue by opening file://usr/share/doc/python/FAQ.html when running Firefox in Safe Mode. However, I did not get the warning when opening about:blank.

$ dpkg-query --status firefox
Package: firefox
Status: install ok installed
Maintainer: Ubuntu Mozilla Team <email address hidden>
Architecture: amd64
Version: 47.0+build3-0ubuntu0.16.04.1
Xul-Appid: {ec8030f7-c20a-464f-9b0e-13a3a9e97384}

$ lsb_release -a
Description: Ubuntu 16.04.1 LTS