Most estimates (based on useragent strings and other methods) show that IE has a market share of around 85%.
Mandating that a company uses an insecure and non-compliant browser (and thus preventing them from replacing IE with FF, for convenience reasons) is not a wise decision. Cross-platform HTML isn't that difficult if you follow the accepted standards and make a few tweaks for IE.
Also, Visual Studio is a development environment, not a language. In any case, a web application should always be easily tuned to provide output to any browser, however you do it - separation of logic from presentation, anybody?