I was trying to log in into my Yahoo mail account last week and I've noticed a strange thing: the page was reloading on and on, and I was practically unable to type in my id and password. I found up quick the cause of my problem: it was because I had turned of cookies in my browser. Most sites where you need to sign in store a cookie to remember who you are, and Yahoo does the same. But the problem with them is that they don't tell the user that his cookies are disabled and if he doesn't figure it out, he risks to have a very unpleasant experience with this.
You could try this experience yourself by turning off your cookies in your browser. In Internet Explorer you can set the privacy options to block all cookies, while in Firefox you can achieve this by unchecking Accept cookies from sites
