Rated 5 out of 5 stars
This is a FANTASTIC addon. I have it on all my computers.
As for the problem with Yahoo mail, be aware that Yahoo is changing (upgrading) the default free accounts. I have 2 Yahoo accounts and it works with one but not the other. The one that fails has been recently upgraded by Yahoo. If you still show the Yahoo avatar on login, you have the old account and WebMail Notifier will work fine. When your avatar disappears, Yahoo has upgraded your account and WebMail Notifier probably will not work.
Again, this is a fantastic program. Thank you Byungwook Kang :-)