Are the certificate settings in Options/Preferences still default (ask)?

• Options/Preferences -> Privacy & Security -> Certificates

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 also try to rename/remove pkcs11.txt and secmode.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

Thank you. Unfortunately, after following your instructions, the certs still don't appear. They are working on the same site with Vivaldi and Opera latest versions.