A website that doesn't support Firefox is a website that doesn't adhere to the W3C standards. Simple as that. It's like a flash drive without a USB port. Useless trash.
Anyone who knows about the existence of Firefox and still uses IE is an idiot.
When you say "advanced scripts that only IE can handle", you really mean "badly written, malformed scripts that only run on IE because every other browser supports the standard form of the language".
Any javascript or AJAX application that doesn't work in any browser other than IE must have been coded by a monkey with a brain deficiency. Firefox handles standard javascript much more efficiently than IE, and implements the standards of the language such as XMLHttpRequest(). (Fixed in IE7, but a fair chunk of the market still uses IE6)
IE also handles CSS incorrectly, with IE6 not supporting many CSS2 standards such as min-height, while IE7 has limited support for most CSS2 standards, it is far from implementing the entire specification.
It has been proven that Opera and Firefox are much more secure than IE, and both support the standards that Microsoft continue to ignore. IE is killing the internet.
So no, I will not "close Firefox" (why close it anyway?) to load your piece of shit which you call a website.