"We can't say whether Firefox is a safer browser or not," said Henry Levy, one of the two University of Washington professors who, along with a pair of graduate students, created Web crawlers to scour the Internet for spyware in several 2005 forays. "But we can say that users will have a safer experience [surfing] with Firefox."

In May and October, Levy and colleague Steven Gribble sent their crawlers to 45,000 Web sites, cataloged the executable files found, and tested malicious sites' effectiveness by exposing unpatched versions of Internet Explorer and Firefox to "drive-by downloads." That's the term for the hacker practice of using browser vulnerabilities to install software, sometimes surreptitiously, sometimes not.

"We can't say IE is any less safe," explained Levy, "because we choose to use an unpatched version [of each browser.] We were trying to understand the number of [spyware] threats, so if we used unpatched browsers then we would see more threats."