It all starts to sound like an AIDS awareness commercial from the '90s—when you browse on one website's server, you're browsing on every server that has been in contact with. And indeed is the result. A hacker can hack one website and spray malicious code all over the Internet, compromising millions of PCs with one attack.
Mozilla's new policy also involves whitelisting known safe websites and probably holds the same potential for conflict of interest. But with the web becoming more interconnected all the time and with the rise of social networking, servers are swapping code all the time now and a system like the one which Mozilla is planning to implement will be necessary to stop malicious hackers using XSS to attack PCs.