It seems more and more people are coming with this issue. Firefox security is usually not that intrusive. While it might interfere at times, nothing at such extremes to my knowledge or at least not what I experienced. It could be that you have a virus or some form of malware running that wants to access your credentials. If you are continuously uploading or downloading shady files through Messenger that could also be an issue.

My recommendations would be, just in case, do a scan with your anti-virus and anti-malware software, check if there aren't any issues. If there are, they should clean it up relatively well. If there aren't, try taking a look at your add-ons and any other custom browser modifications, that can sometimes cause issues. Hope this helps.

facebook currently seems to have issues with that page, please report it to them: https://www.facebook.com/help/326603310765065

The presence of mixed passive content like images coming over an open connection shouldn't prevent from from accessing the page. This is a problem with the website that has images embedded and Firefox informs you about this, but doesn't block these images unless you would toggle the related pref. If you want to see more details then check the Web Console.

• https://developer.mozilla.org/Tools/Web_Console

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/en-US/kb/mixed-content-blocking-firefox
• https://support.mozilla.org/en-US/kb/control-center-site-privacy-and-security-firefox

• https://developer.mozilla.org/Security/MixedContent