Otherwise, Yahoo is temperamental, much because Yahoo seems to constantly fuss with their login procedure... sometimes though, it can help to delete you account definition in X-Notifier, restart the browser and redefine the account. (Restarting the browser in between deletion, redefinition... helps me clear up miscellaneous problems sometimes.) Do you have multiple accounts with Google or Yahoo?
After about over a month not
After about over a month not working, it finally started to.... I had autologin disabled/unchecked. Might it have been because of that?
Edit: Aand it appears Yahoo isnt being checked again.
Troubleshooting
First, have you tried the steps in the following post?
http://xnotifier.tobwithu.com/dp/node/4
Otherwise, Yahoo is temperamental, much because Yahoo seems to constantly fuss with their login procedure... sometimes though, it can help to delete you account definition in X-Notifier, restart the browser and redefine the account. (Restarting the browser in between deletion, redefinition... helps me clear up miscellaneous problems sometimes.) Do you have multiple accounts with Google or Yahoo?