The Discussion Tool can cause Internet Explorer to crash when using the new Yahoo Mail.
To fix this, look for a yellow box with the lower corner turned up along the top of the browser in the Internet Exlporer toolbar.
Click it. While clicking, look at the bottom of your browser window. Does a lower toolbar open with discussion options? If so, click the X on the far left of that toolbar and then try logging in again. This should solve the issue.