hello zompey, a possible solution depends on different factors:

• what is the error code shown under technical details on the error page?

in case the error code equals sec_error_unknown_issuer, please attempt to add an exception on the bottom of the error page & inspect the certificate (see the screenshot attached for instructions):

• which issuer information does the certificate contain?

thank you!

"This Connection is Untrusted" error message appears - What to do

You can 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

thank you cor-el :) it was a Malware and now it's working fine (Malwarebytes' Anti-Malware)

thanks agin cor-el and philipp :)

I have the same problem, even with Trusted sites such as Google.com I did a full virus/malware scan with Kaspersky (all definitions up to date) and found no malware at all. Also, I'm using a brand new computer that I just turned on for the First time today (running Windows 7). I'm running a brand new installation of Firefox, using the latest version plus all updates.

I've been a Firefox user for over 10 years, although this is a new version of Firefox for me. I've never had this problem with previous versions of Firefox, but I'm wondering if this newer version has some settings that I didn't notice that could be responsible for this continuing error. Could I have set my security settings "too high"? I don't want to completely disable the detection of trusted/untrusted sites, though.

It was a silly malware problem. Luckily I did a search in my 'programs and features' in windows and uninstalled a program I did not recognise. It was called Robot tab with an image of a rocket. It was only a few kilobytes, so I guessed it was a malware I accidentally downloaded when installed a free software i got from the net. Uninstalled the little monster and my firefox is as good as new!!

I just opened FireFox for the first time this year and it says Google.Com is not trusted. I have scanned my PC and found no problems. I am always running security software. All my other browsers do not do this. Hoping to find other reasons why Firefox saying Google is not a trusted site?

I alse had this horrible annoying problem and I just managed to fix it.

I'm using Replay Media Catcher (it captures video/audio from the internet), and I found out that when the program is open everything is fine, so I searched google and found this: https://replaymediacatcher.zendesk.com/hc/en-us/articles/202520104-Why-did-I-lose-internet-connectivity-after-using-RMC5-

And so what fixed it for me was going to: Internet Options >> Connections >> LAN settings >> Unchecking the third option "Use a proxy server for your LAN" (here's a screenshot to help - http://oi59.tinypic.com/206hml0.jpg) And also disabling the option "Monitor HTTPS/SSL" in Replay Media Catcher.

I hope this helps :)