Please also make sure that your computer is not affected by superfish: https://mana.mozilla.org/wiki/display.../Lenovo+Superfish+malware+check+and+removal

It is possible to remove the stored cache for yahoo to clear any corrupt cookies or pages. However it will remove all data associated with the site:

1. Go to about:permissions in the url bar
2. Search for yahoo
3. Click Forget this Site

Did this help when you logged back in?

This is an Android tablet not Windows device. Firefox 35 has been out for 6 weeks we have not made any changes.

You can try using a private browsing tab to log in to Yahoo. If that still does not work then the problem is that Yahoo made a change to their website.