This doesn't directly address your problem, but...I started getting negative vibes on my Yahoo account as well, and did some searching around.
Its possible that we don't need to use the Webmail extension to get around the restrictions to POP download mail from Yahoo anymore. See:
http://kb.mozillazine.org/YahooI disabled the extension for Yahoo in Thunderbird and switched the settings to those detailed in that article, and was able to download email from Yahoo to Thunderbird without a problem. Only thing to watch out for is that it will download *all* email that's in your inbox, so if you've been letting messages sit in there for months and months as I was, you could have a lot of messages getting transferred the first time.
This may also be the case for Hotmail (
http://kb.mozillazine.org/Hotmail) and GMail (
http://kb.mozillazine.org/Gmail)
Hope that's helpful.