Firefox pushed a new version of their browser, 52.0 which has the added feature of warning the user of unsecure connections. It also seems to prevent logging into the game. There is another thread about this very discussion and many of us solved the problem by reverting back to an earlier version of Firefox and then preventing FF from automatic updates. I was unable to log into the game using 52.0 but can get on now with 51.0. Hope that helps.