ForceTLS
ForceTLS allows web sites to tell Firefox that they should be served via HTTPS in the future; this helps secure you from accidentally negotiating an insecure session with certain sites. Force-TLS is also compatible with Strict Transport Security. ForceTLS is an adaptation of the ForceHTTPS protocol by Collin Jackson and Adam Barth, which supports a simple HTTP header in forcing automatic connections to HTTPS connections in the future. This helps prevent cookie theft and other man-in-the-middle attacks when you're using an insecure network. See also https://addons.mozilla.org/en-US/firefox/addon/force-tls/
