Out of interest what security software are you running - ie: antivirus and firewall?
I had the certificate problem with Firefox, I think it was when I reinstalled ESET. I eventually discovered that you can export the certificate from ESET and import it into Firefox and the secure connection issue was resolved.
If you do happen to be running ESET, this is how to export the SSL certificate with v9.
1. Open ESET and press F5 to go into Advanced Setup.
2. Click on Web and Email from the list on the left.
3. Click the + next to SSL/TLS, scroll to Root Certificate, then click View Certificate.
4. Click the Details tab then click the Copy to File... button.
5. Save the certificate as a DER type in a handy location, such as your desktop.
Now if you want to import the an SSL certificate into Firefox, do this.
1. Go in to Firefox options, click Advanced then Certificates.
2. Click the Authorities tab then import.