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

http://www.ehow.com/how_11385212_troubleshoot-reset-connection-firefox.html

https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

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

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

This Connection is Untrusted is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.

What security software do you have?

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• click "Advanced" to expand the error message
• click "Add Exception" to open "Add Security Exception"

If this isn't possible then open "Add Security Exception" by pasting this URL in the location/address bar and paste the URL of the website (https://xxx.xxx) in it's location field.

• chrome://pippki/content/exceptionDialog.xul

Let Firefox retrieve the certificate -> "Get Certificate"

• click the "View" button and inspect the certificate
  check who is the issuer of the certificate

You can see details like intermediate certificates that are used in the Detail tab.

Who is the issuer of the certificate?

cor-el said

What security software do you have?

Avast... i already take off the option "Scan for HTTPS sites" from avast, didnt resolve too.

I added the https://www.google.pt website and it says unknown identity

Did you check who is the issuer of the certificate?

the issuer is unknown, see the image below (sorry for being in portuguese)

The issuer is well known and is Fiddler.

FiddlerRoot (DO_NOT_TRUST_FiddlerRoot) usually indicates unwanted software being installed named similarly to BrowserSafeguard, BrowserSafe, SafeGuard See /questions/982532#answer-520145

So technically i have a unwanted program or a malware virus on my pc?

ok, time for some searching and destroy... i'll be back soon to report

Ok so, i'm currently scanning with all the anti-virus/malware/spyware programs you just provided me. At this moment only SUPERantispyware have discovered some adware tracking cookies, and malwarebytes blocks something when i run firefox (image below) and after blocking i can search in adress/search bar with no problems. Theres something here... lets see if the programs detects something more

Done scanning. Microsoft Security Scan found an trojan, Kaspersky Virus Removal Tool found 3 and Malwarebytes found 7 (sorry forgot to take a screenshot on this one) but all good, restarted the machine and the problem on the firefox is resolved :D

Thank you guys