Ensure Firefox's content sandboxing (Level 2) works

It’s probably going to be enabled in Tor Browser 7.0.7 (that we likely won’t ship) and 7.0.8 (that should in Tails 3.3): https://mailman.boum.org/pipermail/tails-dev/2017-September/011731.html

Original created by @intrigeri on 14751 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information