I am getting a certification issue when trying to visit 2 specific sites which are some of the most visited sites in the web: https://www.google.com and https://www.youtube.com The error it shows when trying to visit these 2 pages, is SEC_ERROR_UNKNOWN_ISSUER. I have tried to troubleshoot it and follow the solutions recommended at: https://support.mozilla.org/en-US/kb/error-codes-secure-websites#w_the-error-occurs-on-multiple-secure-sites however none of the solutions described in this guide worked to solve my problem. In that guide, one of the suggested solutions is to scan my system for any malware. I used the following: Windows Defender Maleware Bytes AntiMalware Karpersky TDSSkiller however the results are all clean. In the guide, it suggested that I check the clock as well. The system clock however is synchronized. Last, I have resetted Firefox Profile but this didnt fix the issue either. I shall note that the issue appeared yesterday and so far nothing helped fixing it. Not even the system restarts. The issue about visiting these two sites, is limited only to Firefox. I can access https://www.google.com and https://www.youtube.com without problems using Google Chrome and the Opera Browser. Also, Google Mail and Google Translate work fine on Firefox. These two sites can be accessed with Firefox without causing any SEC_ERROR_UNKNOWN_ISSUER errors.