Your issue is one echoed by many using Yahoo.. Your best bet is to set-up your browser (any browser used) so that you go to another home page. Choose MSN,or wherever, but not Yahoo! Do this and then go to Yahoo mail log-in and make a book mark to it in your browser. Do this and you will not see the issue you are seeing. My suggestion is my belief you are now going to your Yahoo home screen in your browser and then click on an icon there to get your email.
For the record, I do not recommend Yahoo / Yahoo email. Both are dinosaurs at best