Firefox displays “This Connection is not Secure. Logins entered here could be compromised” warning messages when sites don’t protect their login pages with HTTPS.
The idea behind the feature is to display a visual reminder to Firefox users that the data that they enter into a form is not protected when they hit the login or submit button on websites that don’t use HTTPS.
While that is a handy reminder for many inexperienced Firefox users, experienced users may not find it super handy to have.
The main reason for that is that you can look at the page address, or the lock icon, displayed in the browser’s address bar to see the same thing. If there is a red strike-through lock icon, and if the site is not using https, then anything that you enter on the site and submit is not encrypted and thus readable.
Disabling the contextual warning
Here is how you disable the “this connection is not secure” warning in Firefox:
Load about:config in the Firefox address bar and hit the Enter-key.
Search for security.insecure_field_warning.contextual.enabled.
Double-click the preference
The default value of the preference is true, which means that the feature is enabled and that Firefox will display warning prompts when you activate insecure login fields. If you set it to false, those warnings are not shown.