Same here ( Thunderbird 6, Webmail 1.4.2, Webmail Yahoo 1.5.3 ), thats cool, ... Thunderbird 5, Webmail 1.4.1, Webmail Yahoo 1.5.0 worked well.
I attached my log (zip), this may help - but I got "An error (#339) occurred while communicating with the server", try to post after removing the attachment.
