The single reason enterprises with Windows networks prefer Internet Explorer over Firefox and other browsers: Group Policy.
For those of you who might be new to Group Policy, here is a quick background. Let’s first assume you use an Active Directory environment to administer the computers in your corporate network. If that is the case, Group Policy provides a wide set of policy settings to manage IE8 after you have deployed it to your users' computers. These settings are locked down and cannot be changed by users, as they are always written to a secure tree in the registry.