soundesigner said

No luck in "safe" mode, with virus protection turned off, or standing on my tippy-toes.

standing on my tippy-toes works for me sometimes  :-)

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

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

Went down the list (many of which I had tried before), still no dice. I'm running AVG (not on the list) and even after disabling, symptoms persist. Ready to do a clean install.

AVG has a couple of features that can intercept secure connections from Firefox, but it's hard to tell from that rather unexplanatory error page whether that is relevant.

Is there anything special about how Firefox connects? You could check whether you have Firefox set to use a proxy server in settings, and switch that off as a test. You can do that on the Options page:

"3-bar" menu button (or Tools menu) > Options

In the left column, click Advanced. On the right side, click the "Network" mini-tab and then the "Settings" button.

The default of "Use system proxy settings" piggybacks on your Windows LAN connection settings, but you could try "No proxy" to see whether that helps.

soundesigner said

I've tried the various "fixes" in other posts regarding "Secure Connection Failed" with no success. Every time I try to log in to a site requiring username and password, I get nowhere (including this very forum)! The only thing that works is to launch Firefox as administrator, which as another post said, is only a workaround.

I wish I knew what Windows did differently when you launch a program as administrator. Could it be bypassing parental filtering?

On Windows 10, Firefox not being set up to work with the parental control software Microsoft Family Safety is a known cause of blocked secure connections. To test by turning it off, see: http://windows.microsoft.com/en-us/wi.../turn-off-microsoft-family-settings)

No family settings associated with this computer and I have gone through every suggested fix, article, YouTube video, and answer written on the back of a napkin to no avail. Something happened with either a Windows update, an AVG update (although I shut that off temporarily), or a Firefox update that changed something well outside my understanding.

I'm so used to Firefox that I hate to switch browsers, but I have to be able to get my work email and log in to various sites. I have spent way too much time trying to fix this and real work has fallen behind.

Thanks, FredMcD and jscher2000.