urgh
Given the demands on Browsers to provide better multimedia and interactivity, browsers attract more holes than ever before. AJAX exploits have only just begun as well, for all browser variants.
I disagree with Zombini though (sorry), personally I have had to force remove spyware from 3-4 Norton PC's (Smitfraud, trojans, etc), I installed Comodo instead and the users have had no problems.