No green lock in the address bar? Firefox will notify you before entering your password.
Forbes Tech explains:
Keeping your data secure online isn't something you can do on your own. You need help, particularly from the people who make the software you use. Mozilla knows that as well as anyone, and that's why they made this small—but very important—change to Firefox.
When Firefox 52 started rolling out on Tuesday, it brought with it a tweak to the way your passwords are handled. To keep users from entering sensitive data on sites that aren't doing enough to protect it, Firefox 52 will display a warning whenever a password field appears and there's no accompanying green lock icon in their address bar.