In Firefox 76, Mozilla added an experimental HTTPS-Only Mode that could replace add-ons like HTTPS Everywhere. You can go to about:config and set “dom.security.https_only_mode” to True. If you try to open an HTTP-only website, Firefox shows a warning message that no HTTPS is available.
In Firefox 80, released a short while ago, Mozilla introduced a new setting called “browser.preferences.exposeHTTPSOnly.” After setting it to True, you will see new settings in the preferences of Firefox. The HTTPS-Only Mode lets you enable in all windows, only in private windows, or disable it entirely.
If a website only supports HTTP, you can temporarily or permanently add the website to a list of exceptions. For the exceptions, the global settings of the HTTPS-Only Mode are ignored.