Per Yahoo! Tech Support: Use Firefox; Safari hasn't worked properly since 3.2.
Here is the official response, misspellings and all:
"Thank you for contacting Yahoo! Small Business Support.
"I understand you are not able to switch between your Yahoo and business
email account.
"First I would like to confirm that you are using Safari to access your
emails. The reason why you are having this problem is because of the
recent updates that Apple has done on Safari. If you have noticed
recently Safari got updated and the version updates from version 3.1 to
3.2. Which has caused this issue.
"We have a lot of our Customers who use there accounts through Safari and
every one is having the same problem as you do. This is nothing to do
with your account or with the Yahoo! Server as this is something related
to Safari which needs to be looked into by Apple. Apple is already aware
of this issue. As a work around until they resolve it, you can download
Firefox on your computer and use your account through it. As with
Firefox you will not have this problem and you will be able to switch
between your accounts with no problem.
"If you do not have Firefox installed on your computer, please visit the
link below to download Firefox and install it.
http://www.mozilla.com/en-US/firefox/
"I hope I was able to answer your question.
"Please do not hesitate to reply if you need further assistance.
"Regards,