It is possible that your firewall or other security software blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access to install for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

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 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

Just really curious why this would be fine yesterday and not fine 8 hours later. I have never had this problem with Kaspersky and Firefox before

I removed Firefox from the firewall, then added it back again, still does't work. Went to the Security, Action, firewall on the control panel. removed Firefox from the firewall then added it back. Tried that twice with no success at connecting.

Did you try to boot the computer in Windows Safe Mode with network support?

Please un-install Kaspersky and download Microsoft Essentials from the internet. It is Microsoft approved, free antivirus available online. It provides updates weekly. Hope this helps...

https://support.microsoft.com/en-in/help/14210/security-essentials-download

I went to Kaspersky, and got a much easier answer that worked. Here it is verbatim from their support.

You might consider adding this to your database for future reference. 1) Please enable hidden files and folders display.

In Windows XP: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/win_fcab_show_file_extensions.mspx

In Windows 7: http://windows.microsoft.com/en-gb/windows/show-hidden-files

In Windows 8 and 8.1, you can simply press the combination "Windows Key + X" and choose "Control Panel" – Choose “Folder Options” – go to “View” tab and find and check “Show hidden files, folders, or drives” – click on Apply and then OK to close.

2) Launch Firefox and click on the menu icon (the three-stripe symbol in the upper right corner) > Options > Advanced > Certificates > View Certificates > Authorities.

3) Find the Certificate "AO Kaspersky Lab. - Kaspersky Anti-Virus Personal Root Certificate".

If found once: Click on the second line of the entry and delete it by clicking "Delete or Distrust".

If found several times: Delete all entries.

If there is no entry, proceed with step 4.

4) Next, click on "Import..." and go to the folder C:\ProgramData\Kaspersky Lab\AVP17.0.0\Data\Cert

5) Select the file (fake) Kaspersky Anti-Virus Personal Root Certificate.cer and click "Open"

6) In the window "Downloading Certificate" please select the option "Trust this CA to identify web sites." and click on "OK".

7) Close all windows with "OK" and restart Firefox.

Hi GusJones, I think from your original description the support volunteers didn't realize that "any website at all" was limited to "any website using an HTTPS address."

Kaspersky gave you the correct steps to address that issue, and you can mark your last reply as the solution (https://support.mozilla.org/questions/1151751#answer-947078).

For anyone else reading this thread, this article has more information about the problem: How to troubleshoot security error codes on secure websites.