Hi Essexboy,
Infection can be prevented with add-ons like NoScript. However, due to the nature of custom apps in these browsers, I do agree that there is a more open-source and changeable configuration and settings. Since Firefox and Chrome however do allow users to get extentions and add-ons, in which include script protection, the gap in security can be closed quickly.
To add onto my statement, IE9 users would not be able to render this full-fledged CSS3 Animated Page Properly:
http://neography.com/experiment/circles/solarsystem/Which adds onto my theory that IE9 does not support various CSS3 rules.
Furthermore, IE comes pre-installed with all Windows machines. Thus, all users are forced to use Internet Explorer as their first Windows browser. Not fair in the market of browsers, I must say.