Check the date and time in the clock on your computer: (double) click the clock icon on the Windows Taskbar.

Check out why the site is untrusted and click "Technical Details to expand this section.
If the certificate is not trusted because no issuer chain was provided (sec_error_unknown_issuer) then see if you can install this intermediate certificate from another source.

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"

Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".

• Click the "View..." button and inspect the certificate and check who is the issuer of the certificate.

You can see more Details like intermediate certificates that are used in the Details pane.

If "I Understand the Risks" is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".

• Note that some firewalls monitor (secure) connections and that programs like Sendori or FiddlerRoot can intercept connections and send their own certificate instead of the website's certificate.
• Note that it is not recommended to add a permanent exception in cases like this, so only use it to inspect the certificate.

If you are using the feature of ESET that filters your web traffic, this problem could be due to ESET intercepting your secure connections and presenting a fake certificate. If you see ESET as the issuer, you have some options:

(1) Turn off SSL filtering

This would solve the problem instantly, but you would have less protection. Check your product documentation for the specific details, or if you have ESET NOD32 ANTIVIRUS 7 look under:

Advanced setup (F5) > Web and email > Web access protection

Here you can adjust whether HTTPS is scanned. (At least, so it appears from the manual.)

(2) Try ESET automated certificate import

This will allow Firefox to accept the fake certificate signed by the ESET certificate.

These steps are listed on the ESET forum:

• With all browsers and email clients closed, disable SSL scanning (Advanced setup (F5) > Web and email > Web access protection, I think under Protocol Filtering > SSL), OK the change

• Re-enable SSL scanning, OK the change

• Start Firefox and test

(3) Manually import the ESET certificate into Firefox

This will allow Firefox to accept the fake certificate signed by the ESET certificate.

According to the manual, you can export the certificate from ESET under:

Advanced setup (F5) > Web and email > Web access protection, maybe in the section Protocol Filtering > SSL

Look for

View Certificate > Details > Copy to File...

Once you save the file (probably with a CER extension) you can import it using Firefox's Certificate Viewer.

"3-bar" menu button (or Tools menu) > Options > Advanced > Certificates mini-tab > "View Certificates" button

Click the Authorities tab then the Import button.

Thanks a lot Sir.

It worked wonders.