In most managed corporate environments, you do have to run IE. The freedom of installing your own software at will doesn't exist.
a. purchase new hardware for the latest OS from Microsoft,
b. Get all the OS images ready to deploy,
c. Train staff to use all the new applications that have changed UIs due to older versions of the software not working well on the new OS.
Not only this, but as I've said ad nauseum, development costs should reduce when you use something that is updated as often to the latest standard as Chrome and Firefox.
In each case you have a problem: those who use ActiveX will eventually find a total lack of support and updates for these apps, and those apps that rely on non-standard behaviour risk getting broken in even minor updates. Not to mention there will come a time when Microsoft will completely stop supporting the browser version the app is relying on, even for security updates.
As has been pointed out by someone else, Firefox has special long term updates for corporations in restricted environments, so it's not a huge concern in terms of lots of updates being applied every few months.
[1] http://portableapps.com/apps/internet/firefox_portable
(edited to add forgotten link)