Peterdavis said

I have never had a message like this. It appeared out of the blue, and outside any open window. Shortly thereafter there was an un-asked resetting of (I can't really remember) my email server. So I'm apprehensive of some sort of hacking.

Mozilla never sends out any email resets nor would make such alerts unless you choose this option or asked to send a reset. But you should hover the mouse over the email address and see what the actual email address is coming from that would tell you if that was legit email or not from the right sender.

What do you mean by

Peterdavis said

Shortly thereafter there was an un-asked resetting of (I can't really remember) my email server.

You certainly could take some precautionary measures such as restarting your system to close any open connections, reviewing programs that run automatically, and using available anti-malware scanners.