www.softmoc.com uses an invalid security certificate.
The certificate is not trusted because the issuer certificate is unknown.
(Error code: sec_error_unknown_issuer)
You know SoftMoc is a reputable company that you can trust. But the "Add Exception ..." looks scary and you are not sure about doing it.
Here is the safest way to let you use this site worry-free.
(skip this section and go directly to the solution if you don't care about the technical details.)
The reason why you have encountered such an error is that to encrypt their transactions, softmoc.com use a certificate signed by a Chained/Intermediate Certificate from VeriSign (one of the major certificate authorities). This Chained/Intermediate Certificate is named VeriSign Class 3 Secure Server CA - G2. By default, it is not added to Firefox's Certificate Manager. However, if the website were configured properly, it would send the whole certificate chain to Firefox so that Firefox could trust the Chained Certificate. Apparently, softmoc.com is not properly set up.
The good news is that Firefox can automatically add those intermediate certificates it trusts. So if we have ever visited a website which properly uses the certificate of VeriSign Class 3 Secure Server CA - G2, our Firefox would know this certificate is good. Consequently, Firefox would trust softmoc.com. I have done a little search and found the website that correctly uses this missing intermediate certificate.
Go to one of the web page of the famous Sun Microsystems company: https://getupdates2.sun.com/. The web page will prompt you a dialogue to ask for user name and password. We don't want to do anything with this website, so just click the Cancel button. After that, the required issuer certificate is known and added by Firefox.
Now you can enjoy your shopping at softmoc.com.