Dan Goodin at The Register reports that Microsoft's IE 8's Cross Site Scripting prevention feature can be used to create an XSS attack.
IE8 attempts to block XSS attacks by modifying the response, i.e. the content of the web page generated by the web server coming to the browser in response to a request. The NoScript Firefox add-on, takes the opposite approach by modifying the content of the request from the browser to the web server. Here is more information. It appears that this vulnerability is not easily fixed because it's a design flaw rather than a coding flaw.
BTW, NoScript is the second most popular Firefox Privacy & Security add-on.
