Not only is IE the worst browser you can use, all the back end technologies that businesses use (like IE only sites and such) are becoming less and less common. Companies are shifting away from Active X and going with open standards.
Microsoft is really good at one thing, and that is business. They got into all the right places at the exact right time in the 90s during the tech boom, and they came up with technologies to allow big business bring computers to every employee. However, times are changing and companies are now realizing that there are better back end products. Plus, web developers are more common these days, and thus there are more of them. Which means more sets of eyes at more code all improving the web's open standards.
The only reason why anyone will support IE, is because some companies spent multiple millions of dollars building an Active X back end, and is that it comes standard on every Windows box you ever use.
Firefox is actually sort of hard to manage in a corporate environment. You gotta do all kinds of crazy stuff, like import profiles, lock down firefox preferences, password protect certain parts of the browser. That is if you don't want your users downloading and installed 100s of FF extensions on their computer.