I'm running Avast Anti-Virus 5.0.677.
My OS is Windows XP SP3.
I have Comodo Firewall running, but downloading of POP3 from incoming.yahoo.verizon.net works fine with just Comodo enabled. Comodo is not the problem.
I looked at the page you linked to. It's not real clear what they are saying there, but from what I can tell, I am set up correctly. In Thunderbird, under the security settings for the Verizon-Yahoo account that is having issues, the type of security is set to "None" (other options are "STARTTLS" and "SSL/TLS) ... and the Authentication method is set to "Password transmitted insecurely." In Avast, under mail accounts in the expert settings, the servers for the Verizon-Yahoo account are not listed at all. I believe this is correct, no?
Keep in mind when I totally disable Avast, then the POP3 mail from incoming.yahoo.verizon.net downloads without any problems.
When I try to download mail with Avast enabled, this is the error message that Thunderbird gives:
"An error occurred with the POP3 mail server. Mail server incoming.yahoo.verizon.net responded:"
And then right underneath that line there is simply a yellow triangle with an exclamation mark in the center. No error code. Just the symbol.