In this week's stable update to Firefox 83, Mozilla introduced the HTTPS only mode. "HTTPS provides a secure, encrypted connection between Firefox and the website you visit," the option description says. Today, most websites support HTTPS. If you choose to enable the HTTPS only mode, Firefox will upgrade all connections to HTTPS. ".
Although it is not enabled by default, users can enable it in the settings application. This security enhancement ensures that all you visit is an HTTPS site. If you visit an HTTP site after this mode is enabled, it will be automatically upgraded to an HTTPS site. If the site you visit does not support HTTPS, the "secure connection not available" error message will be displayed.
With this mode enabled, all your connections are encrypted and secure. So, you can rest assured that no one can pry into your page content or hack into your site's connections to steal passwords, credit card information or other personal information. This is useful when you're using public WiFi, a connection where you can't determine the integrity of your Internet connection.