You can see a special padlock at the left end of the location/address bar. *a padlock with a strike through means that mixed active content is blocked. *a padlock with an exclamation mark attached means that mixed passive content (e.g. images) is present, but not blocked. The shield icon, previously used for mixed active content, is now used for Tracking Protection. *https://support.mozilla.org/kb/mixed-content-blocking-firefox *https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox *https://developer.mozilla.org/Security/MixedContent ---- This can happen if there are (embedded) images present on the page coming via an open HTTP connection. You can check that in the Web Console ("3-bar" menu button or Tools > Web Developer) and search (filter) for mixed. *https://developer.mozilla.org/Tools/Web_Console