Is there a problem going to other sites? Start '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in Safe Mode]''' {web Link} by holding down the '''<Shift><br> ''(Mac=Options)'' ''' key, and then starting Firefox. A small dialog should appear. Click '''Start In Safe Mode''' (not Refresh). Is the problem still there?

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content. See also: *https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER ---- If you can't inspect the certificate via Advanced (I Understand the Risks) then try this: Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate: *<b>chrome://pippki/content/exceptionDialog.xul</b> In the location field of this window type or paste the URL of the website with the https:// protocol prefix (https://xxx.xxx). *retrieve the certificate via the "Get certificate" button *click the "View..." button to inspect the certificate in the Certificate Viewer You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then please attach a screenshot that shows the Certificate Viewer with the issuer. ----- Do a malware check with several malware scanning programs on the Windows computer. *https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware Please scan with all programs because each program detects different malware. All these programs have free versions. Make sure that you update each program to get the latest version of their databases before doing a scan. *Malwarebytes' Anti-Malware:<br>http://www.malwarebytes.org/mbam.php *AdwCleaner:<br>http://www.bleepingcomputer.com/download/adwcleaner/<br>http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml *SuperAntispyware:<br>http://www.superantispyware.com/ *Microsoft Safety Scanner:<br>http://www.microsoft.com/security/scanner/en-us/default.aspx *Windows Defender:<br>http://windows.microsoft.com/en-us/windows/using-defender *Spybot Search & Destroy:<br>http://www.safer-networking.org/en/index.html *Kasperky Free Security Scan:<br>http://www.kaspersky.com/security-scan You can also do a check for a rootkit infection with TDSSKiller. *Anti-rootkit utility TDSSKiller:<br>http://support.kaspersky.com/5350?el=88446 See also: *"Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

I have upgraded to WIN 10 and this solved my problem and can now access the web via Firefox.

That was very good work. Well Done.