This is the same problem as https://support.mozilla.org/en-US/questions/1142423 but I now have a much clearer description of it. Firefox 49.0.2 running on Windows 7, all extensions disabled. I've cleared all history (cache, site preferences, etc.) I have an HTTP proxy configured (Manual Proxy Configuration, "Use this proxy server for all protocols" checked). If I browse to an HTTPS site after starting Firefox before browsing to an HTTP (non-SSL) site the status bar quickly changes between "Looking up (host)...", "Connecting to (host)..." and "Waiting for (host)..." several times and then shows the "Secure Connection Failed" page, as in the screenshot on https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message This happens on every HTTPS site I try, e.g. https://support.mozilla.org/, https://www.google.com/, https://www.pandora.com/, https://www.ycombinator.com/ (note that this last one does not use HSTS). My proxy server requires HTTP authentication and Firefox does not even prompt for a username and password at this point. I control the proxy server and can see in its logs that there are no connection attempts yet. If I browse to an HTTP site and enter the proxy credential when prompted I can then browse to HTTPS sites as normal. It doesn't have to be the same site, e.g. I can browse to http://www.yahoo.com/ and then https://www.microsoft.com/ will work. However, if I cancel the proxy credentials prompt the issue continues. It takes a successful HTTP connection to make HTTPS work.