Future versions of Firefox may introduce HTTPS only mode, which means blocking access to unsafe websites.In the latest Firefox 76 nightly version, Mozilla has introduced an experimental feature, which will be available to all users in the stable version in the next few months if all goes well.
Browser manufacturers, including Mozilla, have been promoting HTTPS for a long time. Although most websites have upgraded and enabled the security protocol, some websites still use HTTP. After HTTPS only mode is enabled, Firefox browser will not load HTTP website.
The browser will first attempt to access the server to get the HTTPS link, and if the version does not exist, an error message will be provided to the user with the content "secure connection failed". The way to enable is to install the latest Firefox nightly version. On the about: config page, enable the flag dom.security.https_only_mode. HTTP pages will be forced not to load again when enabled.