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 is likely a problem with image that are embedded in the email and that come via an open HTTP connection. Firefox considers these images as mixed content. You should see warning messages in the Web Console (3-bar Menu button or Tools > Web Developer) about this content. *https://developer.mozilla.org/Tools/Web_Console