Sure, Firefox may win in sheer number of add-ons, but many of the customizations you'd want to download for Firefox are already a part of Internet Explorer 8 – right out of the box.
The big difference is that with Firefox (I can't speak for Chrome) you ONLY add what you want and need. IE8 is greatly bloated, most likely carrying around all these 'add ons' that MS thinks someone may need at some time for some thing!