Update: It's working for me now with the mandated settings. Might have been a temporary issue the other day. A couple of points of order, here:
Folks, be nice to the help here, on the phone and in person, even when they're wrong/misinformed. If you've really caught flies with vinegar before, well, I won't presume to lecture you. Secondly, I really don't think there was a Thunderbird issue in play here. If another POP3 client worked but Thunderbird didn't I would agree, but just because the webmail works doesn't automatically mean POP3 clients are good to go.
But yeah, all systems seem to be go. (knocks on wood)