I have this exact same problem when trying to log in to my paypal account since the last FF update, REALLY irritating.

To "fix" it go to tools menu > clear recent history and select cookies and cache, then click "clear now". Now go back to your yahoo account, it should work fine.

Unfortunately you are now also logged out of any other sessions you had going at the time, gmail, forums etc. and have to log back into them all. I really hope firefox fixes this issue or I'll have to go back to Safari.

This issue can be caused by corrupted cookies. You only need to remove the cookies from sites that cause problems. You may also need to clear the cache.

• "Clear the Cache": Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Firefox > Preferences > Privacy > Cookies: "Show Cookies"