This is an issue with MSN and not Firefox, and hence it's probably possible to switch back through your settings on the page. However, if you can't find that way, you can always remove the cookies for the site. Click Tools on your menu bar and goto Settings. Under Security you will find a link to remove single cookies, and if you click it you can remove cookies from msn.com and/or any other MSN sites that cause this problem. Once you have removed them, goto msn.com again and you should stay on that site. Make sure not to enter asus.us.msn.com again, as you will have to redelete your cookies if you do so. (Do note that I'm not using the English language in Firefox, and my details may not be 100% correct to the word in the details of how to remove the cookie, but you should get a hang of it.)