jworks said

Yesterday I got some kind of weird error message on Tbird. I just closed Tbird and reopened and that solved it. Now, today I notice its not downloading email that I know are out there. How to fix this and why did it happen?

Yesterday you got a message that probably said what the issue was. Today you want someone to figure out for you what it said.

Are you using AVG? It is having all sorts of issues with POP mail accounts.

That's right, that's why I came here. No I don't use AVG. I have McAfee. The first message was such that I couldn't understand it and I didn't write it down. If the restarting and it popped up again I would have. But the message didn't show up. However, TB then went through a time of downloading hundreds of old emails, which I promptly deleted. Sometimes in the distant past I've seen a corrupted email cause unusual problems. So I deleted all I didn't need. Now its working as it should.