It's because yahoo uses a special script or code to prevent browsers from remembering your password, part of their overportective security crap. (A site should never force you to sign in again, if someone's too stupid to log out when they know they're on a public computer, that's their problem. The rest of us shouldn't have toi suffer for their stupidity.) To get around it, you need to use a bookmarklet. Go here:
http://www.squarefree.com/bookmarklets/forms.html#remember_password and add the bookmarklet to your bookmarks. Now go to yahoo, log out, go to the log in page, go to the bookmarklet (at this point it should say something like "do not remember has been removed from X forms"), now log in & it should prompt you to save the password. You only need to do this once, from then on the browser will automatically fill in the password.
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"People who live in glass houses ....should close the friggin curtains!"