NSS tested Internet Explorer 9, Chrome 12, Opera 11, Firefox 4 and Safari 5 during 14 days, May 27 through June 10.

The test included 1,188 URLs that NSS had verified contained malware, and an average of 86 new ones were added each day.

Chrome's score represents a big improvement over its performance in last year's test when it scored just 3%. Firefox lost ground against last year's test when its score was 19%.

As soon as NSS discovered new malicious URLs, they were added to the test, then NSS measured how long it took the browsers to recognize them as such, only recording the calculation if the browser actually recognized a URL as malicious. If it never recognized a given malicious site, it wasn't used in the calculation.

Internet Explorer blocked sites on average within .56 hours. Safari was second with 4.9 hours, then Firefox (6.07 hours), Chrome (17.64 hours) and Opera (18.39 hours).