Future versions of Firefox may introduce HTTPS-only mode, which is to block access to unsecured websites almost all the time. In the latest Version of Firefox 76 Nightly, Mozilla introduces an experimental feature that, if all goes well, will be available to all users in the next few months.
Browser vendors, including Mozilla, have long been pushing httpS, and although most sites have now upgraded the security protocol, some sites still use HTTP. When HTTPS-only mode is enabled, Firefox no longer loads HTTP sites.
The browser first tries to access the server for an HTTPS link, and if the version does not exist, it provides the user with an error message that says “Safe connection failed.” The enablement is to install the latest firefox Nightly version, and enable dom.security.https_only_mode this Flag in the site: config page. The HTTP page will be forced to no longer load after it is enabled.