AT&T and other websites don't load when using Firefox but work just fine with IE
After updating Firefox to 47.0.1, I am again unable to load the AT&T website (www.att.com) so that I can logon to the account for my cell phone. No error message is displayed - Only a blank area where the page is supposed to be displayed. The website loads and works fine with IE. I have checked all of the things listed in your articles about websites not loading, and none of the suggestions have helped. The problem started around the time I updated to 44.0, and appeared to gone away after moving to 47.0. Only to come back with 47.0.1.
Solution choisie
I checked out all of the suggestions made by FredMcD and found that none of them resolved my problem. Then I noticed additional icons on the left side of the area that has the website's address. One of those icons is a circle with an "i" which I assumed stood for info. When I clicked on it, I saw saw an entry called "Tracking Protection" which said "Firefox is blocking parts of the page that may track your browsing." Below this message is a bar labelled "Disable protection for this session." When I click on the bar, the webpage appears. I found that I had turned on a Privacy Option called "Use Tracking Protection in Private Windows" which I have now unchecked. This change fixes my problem across Firefox shutdowns and start ups.
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Use this link to check that your plugins are up-to-date. If any are listed as unknown or research, do not worry about them.
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.
Solution choisie
I checked out all of the suggestions made by FredMcD and found that none of them resolved my problem. Then I noticed additional icons on the left side of the area that has the website's address. One of those icons is a circle with an "i" which I assumed stood for info. When I clicked on it, I saw saw an entry called "Tracking Protection" which said "Firefox is blocking parts of the page that may track your browsing." Below this message is a bar labelled "Disable protection for this session." When I click on the bar, the webpage appears. I found that I had turned on a Privacy Option called "Use Tracking Protection in Private Windows" which I have now unchecked. This change fixes my problem across Firefox shutdowns and start ups.
That was very good work. Well done.