[fertel]

In my small subset of the enterprise market, amongst my clients (large financial services firms) IE usage is 100%. Unfortunately I don't see this changing any time soon.

[igrekel]

It is not completely impossible but there are 3 big reasons they stay with IE that I heard of.

Transparent Identification/Authentication, its is possible to support the protocols with Firefox but it is not as transparent.

ActiveX like stuff... sadly several enterprise applications use that.

Justifying the tests and rework of switching to a different browser.