Try to rename the cert8.db file (cert9OLD.db) and remove a possible cert_override.txt file in the Firefox profile folder with Firefox closed. *https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean *https://developer.mozilla.org/Cert_override.txt If this has helped to solve the problem then you can remove the renamed cert8OLD.db file. Otherwise you can undo the rename and restore cert8.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 <b>about:profiles</b> page. *Help -> Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/Profiles