There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

http://kb.mozillazine.org/Error_loading_websites

https://www.bing.com/search?q=web+site+access+denied

Forgot to ask. Is this on many websites, some, a few?

Please provide public link(s) (no password) that we can check out. No Personal Information Please !

Hello,

Yes it is on multiple websites. E.g.

https://www.winthrop.edu/

https://forums.frontier.co.uk/

I have Bitdefender, but this issue is confined to my main PC. Bitdefender doesn't cause issues on my tablet.

I had no issue with those links.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Start your Computer in safe mode with network support. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

Neither of those worked.

I called for more help.

You can check the connection settings.

• Options/Preferences -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• 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/en-US/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:
  https://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  https://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  https://www.kaspersky.com/security-scan

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

• Anti-rootkit utility TDSSKiller:
  https://support.kaspersky.com/viruses/disinfection/5350

https://support.kaspersky.com/viruses/utility

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

Hello,

Changing the proxy settings did nothing.

I already have Malwarebytes. I scanned with that and the other programs you suggested and my computer came up clean.

Could the drivers/software from another device be an issue? This started occurring after I got a new set of headphones on Friday. Steel Series Siberia 150, specifically.

For me this looks like an issue with software that acts like a man-in-the-middle and places itself in the connection chain. This can be security software or malware or the possible use of a proxy, so maybe check the network settings and try to reset the router (power off, wait at least 30 seconds before restarting).

Hello,

Well the problem seems to have somehow resolved itself. The websites and Youtube are all working fine now.

Thank you for all your help although I still do not know what the issue was. I'm guessing something updated that corrected this error. I'm guessing it was something to do with security certificate authentication or something, I'm not sure.

Here's hoping. After a few days, if the problem is gone, Please flag your last post as Solved Problem so others will know.

Will do, thanks for all your help. I'll be sure to ask again if it comes back and maybe pinpoint specifically what went wrong.