"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."
Later in the article, there's mention made about how widespread this problem is:
"[...] one of the research's most startling conclusions was the number of spyware-infected sites. One out of every 20 executable files on Web sites is spyware, and 1 in 25 domains contain at least one piece of spyware waiting for victims."