Try disabling email scanning in your anti virus (It does not try and scan mail when you use webmail.)

Disabling email scanning didn't fix it. I can open the Mozilla email client and it immediately tries to connect to the inbound AT&T mail server but then just sits there with no response. If I try to send a message same thing, tries to connect to outbound AT&T server but nothing. One of the options in the support doc said to remove the saved passwords and when the client restarts it will ask you to save your password again. The client never makes a connection to the server so it never has a chance to authenticate. Its almost like AT&T disabled their POP server.

The standard recommendation is try windows safe mode with Networking, When Thunderbird tries to connect to the server, the anti virus in it's guise as a firewall blocks the attempt and it just sits there until it times out. Or it blocks the DNS access so the message says looking up or looking for instead of connecting.

Windows safe mode is the only way to be sure the anti v irus is not actively interfering and even then it can mess up the TCP/IP stack so some thing still do not work.

The other option is your provider has changed their settings.

The plot thickens...I was able to make a successful connection to the server in safe mode. When I go back to normal Windows mode no connection. Why would this only start happening after the Thunderbird update on 4/10? That's the only parameter that changed.

Also - what are crash reports? there's one in the crash report folder dated 4/10.

What anti virus are you using?

Crash reports are diagnostic reports very complex really about what libraries and what programs were running when the program crashed. Nothing private unless you added your email address and whatever explanation to it.

Click it and it will be displayed