Sometimes this is the result of improper changes to the browser's proxy settings. Please check these options to see if it is causing your issue.

Select your 'Options' menu in the Tools dropdown or in the menu button to the right of your address bar. In 'Options', select the 'Advanced' section. In the Advanced section, select the 'Network' tab. In the network tab find the 'Connections' section, select the configure button on the right side of the Connections section. This should open a new window, in this window make sure that the 'No Proxy' selection is highlighted.

With 'No Proxy' selected, try to browse to a webpage and see if the error persists.

Sorry...should have also mentioned that I set Firefox to "no proxy"

It doesn't make sense that Opera is working 100% and Firefox sporadically...

I assumed when you said that you had refreshed Firefox that you meant you used the option in the troubleshooting help menu at about:support. The refresh feature removes a lot of add-ons, but doesn't always fix issues with plugins.

It may be helpful to follow the instructions for disabling plugins here: https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems

Disable your plugins then try to open a page, if pages load after all plugins are set to "never activate" then one of your plugins is likely having issues, that same page contains instructions for removing or updating defective plugins.

Do a malware check with several malware scanning programs on the Windows computer. 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:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware