Did you make any exceptions for that site in the past? *Tools > Options > Advanced > Encryption: Certificates > View Certificates : Servers ---- You can try to rename the file cert8.db to cert8.db.old in the Firefox Profile Folder to remove all intermediate certificates that Firefox has stored by visiting secure websites.<br /> If that helped to solve the problem then you can remove the renamed file cert8.db.old unless you have user certificates that you may want to export first and import them in the new file.<br /> Otherwise you can restore the certificates by renaming (copying) the file back to cert8.db<br /> Firefox will automatically store new intermediate certificates when you visit websites that send them. You can also try to rename or delete the file cert_override.txt (cert_override.txt.old) in the Firefox Profile Folder to remove all permanent exceptions that you have saved. You can use this button to go to the Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Open Containing Folder