I've been using Mozilla Thunderbird email client for atleast six months since installing Ubuntu. I have Firefox installed but have not used it at all recently. Since it's Ubuntu I don't have running antivirus. Suddenly, beginning a day or two ago, every time I open Mozilla I get asked to add a security exception. It says that legitimate sites will not ask me to do this. Something about a certificate belonging to a different site which could mean someone is trying to impersonate this site. I keep canceling it but it always comes back. The location it wants to override how Thunderbird identifies it is outlook.office365.com:993. And there is a button to get certificate. I've not used office375 or outlook within the past several days, though Hotmail is one of the accounts that downloads in Thunderbird. I deleted all mail I've gotten in the past several days taht I don't know exactly what it is and who it came from, and emptied my spam and trash folders. I'm still getting this error. Screenshots are attached. Or, rather, they would be, but they refused to upload. Just jpeg images. Why am I getting this, and what do I need to do to get rid of it?