When a page uses an HTTPS address, but some elements of the page would be loaded on an HTTP address, this is considered a "mixed content" situation. Firefox has two behaviors: (1) Blocking mode: Firefox blocks the most dangerous kinds of mixed content, such as scripts. (2) Advisory mode: Firefox loads low risk mixed content, such as images, and changes the icon to the gray lock with the warning triangle. This article has more information on mixed content and the icons: [[Mixed content blocking in Firefox]]. Now... the key question is whether you should be concerned. When you are working with user-generated content such as email messages or forum posts, the user may never have anticipated that the content would be viewed on a secure connection and therefore didn't think to use HTTPS links. And some embedded images might not be available through an HTTPS connection anyway. In these situations, if you are interested in the content and not exposing personal information in the page, it seems like a reasonable risk to run. You would never expect to see mixed content on a site dealing with highly sensitive information, such as financial or health information. You also would not expect to see the icon on a page that isn't displaying user-generated images. If that happens, it could be flagging a bad add-on that is injected unwanted ads into a page. In those cases, it would be worth investigating the warning in more detail. One way to do that is: * Open Firefox's Web Console in the lower part of the tab using either: ** Ctrl+Shift+k ** "3-bar" menu button > Developer > Web Console * The Web Console should open in the lower part of the tab. Then reload the page (Ctrl+r) and the console will display some warnings and errors. To filter the list, type '''mix''' in the box at the upper right of the list that says ''Filter output'' and you should see specific messages about the mixed content in the page that triggered the warning.