Firefox 52.0 Release (on March 7, 2017) added user warnings for non-secure HTTP pages with logins, now displaying a "This connection is not secure" message when users click into the username and password fields on pages that don’t use HTTPS.

Here’s how to disable Firefox insecure password warnings:

1. Open a new tab, paste about:config into the address bar and hit enter.
2. If you see the "This Might Void Your Warranty" page, click the blue "I accept the risk!" button. Understand we are manually modifying Firefox’s default settings.
3. In the Search box at the top, paste insecure_field_warning.contextual.enabled
4. Double click the setting to change it to "false", to disable Firefox’s insecure password warning.
5. Done! Now when you visit pages with HTTP login forms, the warning will no longer appear.

If you also want to restore autofill functionality, so that your saved login/password automatically populates in an HTTP form, keep the configuration page open and follow the next step.

Optional: In the Search Box on the about:config page, paste signon.autofillForms.http, double click the setting to change it to "true", this will enable autofill.

Sources:

https://www.mozilla.org/en-US/firefox/52.0/releasenotes/
https://blog.mozilla.org/security/2017/01/20/communicating-the-dangers-of-non-secure-http/
https://www.thesslstore.com/blog/disable-firefox-insecure-password-warnings/