The real problem is
that a lot of the web based apps will be 3rd party, so in-house development has to work around their shortcomings.
This is why it's understandable (though still not forgivable) that they've not yet upgraded.
But Firefox? not a sodding chance!
That software has no place on a large corporate network. The Mozilla developers simply don't have a clue as to how a modern (or even vaguely modern) windows network operates.
It stores all its configuration settings in files under each user - a whopping great no-no.
It also stores its temporary internet cache in the wrong application data folder, so if your network uses roaming profiles, your users end up carting vast swathes of useless data around between machines, and clogs up your servers.
As some previous commenters have touched on, group policy management allows extremely granular configuration of every aspect of IE. Because Firefox doesn't use the registry correctly, it's impossible to control or manage in any meaningful way.
The true irony is that IE, no matter which version, is actually vastly more secure than any other browser because of these GP configuration options.
There are literally hundreds of configuration options that you can mix and match depending on what user logs onto what computer.
This means that you can shut IE down into 'read-only' mode for all external websites, and open up every possible 3rd party plugin for your own intranet systems. Combine that with a decent corporate firewall and overall windows lockdown, and nothing malicious is getting through, no matter what vulnerabilities are discovered in the browser.
It's this functionality that makes IE the defacto, and still the only choice for any sysadmin that's got more than a hundred machines to look after.
By comparison, Firefox looks like a tedious and time consuming back door into your systems, so it is of no suprise that they came down so hard on the phone monkeys. (I'm allowed to call them that, I used to be one once upon a time.)
Although, the fact that these guys have enough local user rights to install something, highlights that it's the administrator that should be fined £250 for each machine that goes down because he's too incompetant to secure the machines properly.
I chose Bill as a saint, simply because group policy management has literally saved me thousands of man-hours work! (That's no exageration!)