https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean#w_the-certificate-expired-on-date

Check your system date and time on your Windows install.

Jan 7th 2019, 10:02 - synchronized with ptbtime.ptb.de NTP server....absolutely accurate

Then you got other problems either from the site server is invalid or the O/S can't verify the certificate. Is there a reason you don't have it to sync with a Microsoft based server? That can also invalidate the certificate that fails the O/S check as well.

I am using the german servers since I am in Germany and the PTB supplies the official reference time for Germany.

The page opens flawlessly in Internet Explorer and Chrome

Oh and besides of that no other sites generate problems at the moment so I doubt a general OS problem on my side. Looks more like some flaw in Firefox to me at the moment.

Try to rename the cert9.db (cert9.db.old) file and remove the previously used cert8.db file in the Firefox profile folder with Firefox closed to remove intermediate certificates and exceptions that Firefox has stored. Note that current Firefox releases use a cert9.db SQLite database file.

• https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean

If that has helped to solve the problem then you can remove the renamed cert9.db.old file. Otherwise you can undo the rename and restore cert9.db.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox