Because both of them are better.

Browser: I will always run what I consider the best browser, I don't care what is best I will run that, Internet Explorer used to be the best I used that, before when Netscape used to be the best I used that, before when Mosaic used to be the best I used that, before when lynx used to be the best I used that, Now "FIREFOX!!!!" is the best and I use that I the rest I have for QA.
http://www.mozilla.org/products/firefox/

OS: Because ever since I switched I don't worry about virusus , worms , BSOD , random reboots and it runs very quickly on older hardware and best of all it's FREEEEEE. As with IE the only reason I even use it anymore is QA my development work.
http://fedora.redhat.com/