You may recently heared something about Yahoo! new mail system which is powered by AJAX technology. Yahoo has provided a page for its users to sign up for this new service and try it. After you sign up, Yahoo! may invite you to try it. But here's a short way for you to try it before Yahoo invites you. Just follow these steps:

1- Login to your Yahoo! mailbox.
2- Click on Options at the right side.
3- Click on Account Information at the left side. (You may need to re-enter your password in this step.)
4- In the Member Information section click on Edit link.
5- In the next page you see a section named "General Preferences"; locate Preferred Content, then click on the link provided there (e.g.: Yahoo U.S.).
6- In the next page select Yahoo! United Kingdom for New Setting and click on Finished button.
7- Accept Terms of Service of Yahoo! UK & Ireland.
8- Now, sign out of your account and re-login to your mailbox.
9- You see a page with this message: "It's the New Yahoo! Mail Beta... and you're invited."
10- Click on Try Beta Now yellow button at the right side.
11- Enjoy Yahoo! Mail Beta ;)