Internet Explorer doesn't exactly have it all over Firefox, or even Chrome, when it comes to a robust collection of add-ons that add great features. Still, they exist, and, well, they cause crashes. Most crashes, in fact, according to a Microsoft whitepaper (direct DOC link).
It's intriguing to note, as Ars Technica does, that Flash is not counted among those add-ons that cause a reported 70 per cent of crashes. [Ars Technica]