Have the same problem. Working fine Saturday then nothing Sunday and since then. Thunderbird asks for password with a pop up. After flickering through several URLs (Yahoo and GoogleAnalytics among them) final URL on login page is for tsdtocl.com which seems suspect. Same problem has occurred for a couple of posters on Reddit. I've run several AV checks on my system to see if maybe something had slipped through that changed things but nothing comes up. I've hunted the internet trying to find something similar and the solution but no luck. If you find out anything else, please post. :-)