Several of my users have business-critical websites that only work in Internet Explorer. I can set up shortcuts so that Firefox is used for some sites and IE for others, but that can get confusing.
Also, both Firefox and Chrome are extremely lacking in options for managing user settings. Things that can be set with a group policy for IE require manual scripting for Firefox.
Google isn't ending support for IE, just IE 8. If you're on XP and can't get a newer IE you can certainly use another browser. The number of people locked in IE and XP and are completely cloud based is probably very small.
I didn't mean that to sound like FUD. I work with a number of insurance companies that are on XP/IE7; with a Firefox install requiring approval from a large IT department.
