Firefox automatically protects you from attacks by blocking potentially harmful insecure content on pages. Firefox will display a grey warning triangle or crossed-out lock to indicate that mixed content has loaded on the page.
HTTP is a system for transmitting information from a web server to your browser. HTTP is not secure, so when you visit a page served over HTTP, your connection is open for eavesdropping and man-in-the-middle attacks. Most websites are served over HTTP because they don't involve passing sensitive information back and forth and do not need to be secured.
When you visit a page fully transmitted over HTTPS, such as your bank, you'll see a padlockgreen padlock 
padlock icon in the address bar (For details, see How do I tell if my connection to a website is secure?). This means that your connection is authenticated and encrypted, and thus safeguarded from both eavesdroppers and man-in-the-middle attacks.
However, if the HTTPS page you visit includes HTTP content, the HTTP portion can be read or modified by attackers, even though the main page is served over HTTPS. When an HTTPS page has HTTP content, we call that content “mixed”. The page you are visiting is only partially encrypted and even though it appears to be secure, it isn't. For more information about mixed content (active and passive), see this blog post.
How do I know if a page has mixed content?
If you see the green lock icon in the address bar, the page is secure. If the page had any insecure elements, Firefox already blocked them to keep the page secure. Tap the icon to view more security information, and see whether or not Firefox had blocked any insecure elements.
Firefox will display a grey warning triangle when insecure passive content is present and loaded on a page. If you see this icon, be aware that attackers may be able to manipulate parts of the page, for example, by displaying misleading or inappropriate content, but they shouldn’t be able to steal your personal data from the site.
If you see a lock with a red line over it, Firefox is not blocking insecure elements, and that page is open to eavesdropping and attacks where your personal data from the site could be stolen. Unless you’ve unblocked mixed content using the instructions in the next section, you shouldn’t see this icon.
Advanced users only: unblock mixed content
If you need to unblock mixed content, you can do that by changing your about:config settings. This setting will affect all the pages you visit:
- Go to about:config.
- Change the security.mixed_content.block_active_content setting to false to unblock HTTP content.
- You'll know when Firefox is not blocking potentially harmful insecure content when you see the lock icon with a red line across it:
