Enable HTTPS Only Mode in Mozilla Firefox

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Mozilla plans to introduce an optional HTTPS mode in Firefox 76 that will allow to connect only to HTTPS sites.

Most websites already use HTTPS to improve the security of connections. HTTPS encrypts the connection, which protects it from tampering and also blocks the recording of activities.

Firefox users may soon enable an option in their web browser to allow only HTTPS connections; this is very similar to the way HTTPS Everywhere works. The browser extension attempts to convert unencrypted resources to encrypted resources when enabled, and has an option that blocks all unencrypted traffic.

When enabled, Firefox loads HTTPS sites and resources exactly as before. When sites or HTTP resources are detected, the browser attempts to update them to HTTPS. The site or resource is loaded if the upgrade has worked; otherwise, it is blocked, which can result in sites not loading or partially loading.

What is mixed content and what are the risks?

HTTP is a system for transferring information from a web server to your browser. The HTTP protocol is not secure. So, if you visit a site that is served by HTTP, your connection is open to eavesdropping and man-in-the-middle attacks. Most websites are served by HTTP because they do not need to transmit sensitive information or be secure.

If you visit a site that is entirely transmitted via HTTPS, such as your bank, you will see an Fx70GreyPadlock icon in the address bar. This means that your connection is authenticated and encrypted and is therefore protected against both eavesdropping and man-in-the-middle attacks.

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However, if the HTTPS page you are visiting contains HTTP content, the HTTP part can be read or modified by attackers, even if the main page is served by HTTPS. If an HTTPS page contains HTTP content, we call this content “mixed”. The page you are visiting is only partially encrypted, and although it looks secure, it is not. You can find more information about mixed (active and passive) content in this blog post.

When enabled, Firefox loads sites and HTTPS resources exactly as before. When sites or HTTP resources are detected, the browser attempts to update them to HTTPS. The site or resource will load if the upgrade has worked; otherwise, it will be blocked, which may result in sites not loading or only partially loading.

To enable HTTPS mode only for sites in Firefox

Firefox users using Firefox 76 or later can enable the new HTTPS mode only in the browser as follows

  • Load about:config in the browser address bar.
  • Confirm that you will be careful.
  • Search for dom.security.https_only_mode using the search box above.
  • Set the preference to TRUE to enable only HTTPS connections in Firefox.
  • Set the preference to FALSE to allow all connections (default).

A “Secure Connection Failed” error is displayed by Firefox if a site cannot be upgraded to HTTPS after the preference has been set to TRUE in Firefox settings.

The new “HTTPS-Only” mode works like the “HTTPS Everywhere’s strict” mode because it automatically blocks all unsecured connections. Firefox’s built-in feature does not support fallback mode (which HTTPS Everywhere supports).


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