This article is about a new HTTPS header: Strict-Transport-Security, which force a website to be fetched through HTTPS. This feature will be part of Firefox 4.0. HSTS, specified in an IETF draft, allows sites to specify when they wish to be accessed only over https. These behaviors can be exploited to run a man-in-the-middle attack on a non-encrypted version of the website. To avoid this, you may want to force your websites to be visited through https.”]
Source: https://hacks.mozilla.org/2010/08/firefox-4-http-strict-transport-security-force-https/