With a more full explanation, which I just had to give to a manager;
IE6 was built around 2000, and ignored several significant web standards at the time. IE6 was somewhat of a "lesson learned"; IE7 is better about following those standards, and IE8 is better still.
Firefox was built around Netscape/Mozilla, and importantly, they followed the public standard as much as they could. Firefox largely behaves like Safari, Chrome, Opera, and the tons of tiny-marketshare browsers out there.
Businessperson asks:
So, why support the standard, instead of IE, which is the big kid on the block? Almost all of our customers use IE!
Answer:
Because IE is slowly moving towards the standard, too. If we support Firefox, IE8 is easy, and we probably get IE7 as well with almost no changes. IE6 is the fly in the ointment here.
If we support IE6 - the original proposal - then IE7 is a special case, IE8 is another special case, Firefox is a special case, and so on.
If we can encourage users to move away from IE6, that's our best case scenario. I believe Microsoft officially ended all support - including security patches - for IE6 when Server 2003 SP1 left support, April 2009. Google has stopped supporting IE6 entirely, for example, politely letting users know they need to upgrade "for the full site experience". Sites like IE6NoMore offer a pretty slick CSS popup for those running IE6, giving them a few upgrade options.
But in the meanwhile, since customers do use it, IE6 is here to stay, and it's easiest - and most maintainable - to build to the standards, and hack our way back to IE6 until it's done.