In another question, madrai says:
If you are sure about the service endpoint url, hit it through browser, save the certificate manually in "X 509 certificate with chain (PEM)" format.
How do I know which certificate to choose? In Firefox, under Tools → Options → Advanced → View Certificates → Authorities, you can't see which cert the current tab is using. You just get a huge long list from A to Z. Where should I be looking?