You are not alone. Users have reported getting that AT&T error screen for a few weeks now. One workaround is to open a new private window to access your mail. That bypasses any previously saved cookies for the site, or cached files, and may disable some of your add-ons. Some users said AdBlock Plus and DuckDuckGo Privacy Essentials contributed to the problem. You may want to check the following thread for ideas and commiseration: https://support.mozilla.org/questions/1490607 (unsolved)