See if this help article help you:

• https://support.mozilla.org/en-US/kb/connection-untrusted-error-message?esab=a&s=untrusted+connection&r=1&as=s#w_the-certificate-is-not-trusted-because-the-issuer-certificate-is-unknown

Also:

• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can?esab=a&s=untrusted+connection&r=0&as=s

hello SlamGouveia, could you tell us which issuer information the certificate does contain when you inspect it (see screenshot)?

Well, it doesnt appear anything http://gyazo.com/f220c2037cb2b95da91d06f70a04f4a7.png http://gyazo.com/f28070a113fe405e5e276f9a72fc054c.png

ok, then i'd try to reset firefox and see if this can address the issue...

Did you try to click the Get Certificate button?

That should make the View button active.

Already reseted, everything is still the same. Yes, cor-el, the print I made was exactly after clicking the Get Certificate button

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps.

Do a malware check with some 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: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• 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

Problem Solved. I saw other people having the same problem and i ran the malware detector and now I can use Firefox without problem.

Thanks all of you