You will notice that the address in the address bar starts with an exclamation mark (!):
-Click the exclamation mark
-A sub window will open, click on More Info.
-A window will open, go to Security tab, and click on “View Certificate”.
-Look for Common Name (CN)
-Google the above CN and find a page that allows you to import it into firefox.
For example in the case of the above image the certificate:
VeriSign Class 3 International Server CA – G3
Certificate gets imported by simply visiting: