"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."But the researchers won't say which of the two browsers is the safest. The reason for this is because they used the unpatched versions of both browsers, because they were trying to understand the number of spyware threats.
More details about ths research can be read here.