Mozilla’s Firefox add-on asks web sites to use HTTPS only to request requests via HTTPS. Firefox is working on implementing “ForceTLS” in Firefox with hopes it will reduce occurrences of MITM attacks. ForceHTTPS is intended for both channel encryption, to thwart eavesdroppers, and for server authentication. We built a prototype of the feature that works in a similar fashion to the original design by Collin Jackson and Adam Barth in 2008. Were asking sites to send an HTTP header X-Force-TLS with any securely-transmitted response.”]
Source: https://blog.mozilla.org/security/2009/07/27/locking-up-the-valuables-opt-in-security-with-forcetls/