Does this affect all sites?

There certainly are some sites that instruct Firefox to use HTTPS only (HTTP Strict Transport Security or HSTS), and in fact Firefox includes a list of such sites provided by Google. Firefox will always want to use HTTPS with such sites. Is there a problem using HTTPS with popular sites?

The HTTPS Everywhere extension can force HTTPS connections. Probably you would know if you have this add-on.

There is an experimental feature in Firefox to force HTTPS, but that shouldn't be turned on automatically. You can check here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste https_only and pause while the list is filtered

(3) If the dom.security.https_only_mode preference is bolded and has a value of true, double-click it to restore the default value of false

More info on about:config: Configuration Editor for Firefox.

Note that you should also see this setting at the bottom of "Options/Preferences -> Privacy & Security -> HTTPS-Only Mode".

• https://support.mozilla.org/en-US/kb/https-only-prefs