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Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Content Blocking. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature:

https://support.mozilla.org/en-US/kb/content-blocking

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Restarting in safe mode does not change the behaviour. This also doesn't seem related to external certificates, it's a bug in firefox itself.

I'm pretty sure that we're not allowing screenshots of internal pages. But you're right, it's a bug.

Still you can make a screenshot of this page using the button within the Developer Tools.

TyDraniu said

Still you can make a screenshot of this page using the button within the Developer Tools.

See : https://developer.mozilla.org/en-US/docs/Tools/Taking_screenshots