Enable unsafe-content-script for Firefox

This makes RES work on Firefox 36 (nightly). This will likely break completely in the future, as Mozilla is making security-related changes to UnsafeWindow[1]. For now, however, it makes RES work on the latest Firefox nightlies without issues.

[1]: https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/