You may have ad / mal-ware. Further information can be found in the [[Troubleshoot Firefox issues caused by malware]] article. Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up. -------------- Do the above first. Then, There is security software like Avast, Kaspersky, 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/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 https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER '''This Connection is Untrusted''' is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.