Not-So-Joyful Yahoo! Rated 4 out of 5 stars
I read CPrebble's review of 16 June with great interest because I, too, am having problems with Yahoo, although of a slightly different nature. When I open Firefox, all the initial automatic scans in X-notifier work fine, except Yahoo's. Yahoo accounts remain grayed out, and no e-mail counts appear.
While a minor annoyance, it is not a fatal flaw. If I then run a manual check by clicking the "Check Now" menu entry, lo and behold: the Yahoo accounts are no longer grayed out, and the counts for unread mail suddenly appear.
So for new users who depend on Yahoo, don't give up on X-notifier if initial startup fails. Just run a manual scan; you should then be good to go.
For the benefit of RpD:
As I have stated before, this is a marvelous add-on. It must be a monumental chore to keep current with all the changes made by the numerous service providers, and I have nothing but admiration for the developer of X-notifier.
However, the first three or four times I had problems in the distant past, I did use the forum. Alas: I never had a single reply to any of my posts. In my experience, the forum simply has not produced results.
Since then, I have instead chosen to post reviews so that new users contemplating this add-on might become aware of the types of problems they might encounter so that they can make an informed decision. Sorry if that offends your delicate sensibilities.