One possible cause for having certificate errors on MANY sites is your system clock being set incorrectly (for example, correct month and day, but wrong year).

Another is security software that intercepts and decrypts your secure communications to filter bad content. (Or malware stealing your private information.) Have you installed any new software, or updated your security software, around the time this problem started?

To get to the bottom of it, we would want some more information about the certificate that Firefox is rejecting. By viewing the issuer information, you often can learn what local software is interfering (if it is local software). More on that if you can tolerate a little troubleshooting.

We have seen issues reported caused by Avast and the Avast! Online Security extension that shows in the System Details List.

Check out why the site is untrusted (click "Technical Details to expand that section) and if this is caused by a missing intermediate certificate 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 Fiddler (FiddlerRoot) can intercept connections and send their own certificate instead of the website's certificate.