A Crawler-based Study of Spyware on the Web goes a long way toward explaining why SiteAdvisor must test sites with IE. Cited by Asa Dotzler:
“1.5% of the URLs we crawled in May exploited IE security flaws to install spyware without prompting the user. While this may seem like a small percentage, consider that 1 in 67 Web pages that we examined contained malicious content targeting browser flaws,” while for Firefox “only 0.08% of examined URLs performed a drive-by download installation, but all of these required user consent in order to succeed. We found no drive-by attacks that exploited vulnerabilities in Firefox.”
Amazingly IE still owns 87% of web users (or rather IE-hijacking malware does!)