The checkitlink.com server doesn't send a required intermediate certificate. You can inspect the certificate chain via a site like this: *http://www.networking4all.com/en/support/tools/site+check/ You can download the intermediate certificate from this web page and import the certificate in the Certificate Manager. *COMODO RSA Domain Validation Secure Server CA *http://www.networking4all.com/en/support/tools/site+check/report/?fqdn=checkitlink.com&protocol=https Copy the base64 encoded certificate text that starts with "-----BEGIN CERTIFICATE-----" and ends with "-----END CERTIFICATE-----" to the clipboard after having selected the full text with the mouse. Open a plain text editor like Notepad and paste the certificate text of the intermediate certificate that you have placed on the clipboard in the editing area. Use "Save File as" and set the File type to "All files" and save the certificate text to a .cer file. Select "All files" when saving the file to avoid getting a hidden .txt file extension (.cer.txt) appended. Import the saved certificate in the Firefox Certificate Manager. *Tools > Options > Advanced > Certificates/Encryption: View Certificates > Authorities > Import Do not set any trust bits when prompted as those are only required for root certificates and should never be set for a intermediate certificate like this one.