Usage share, in web browser statistics, is the percentage of visitors to a group of web sites that use a particular browser. For example, when it is said that Internet Explorer has x% usage share, it means that some version of Internet Explorer is used by x% of visitors that visit a given set of sites. Typically, the user agent string is used to identify which browser a visitor is using. The concept of browser percentages for the Web audience in general is sometimes called browser penetration.

Although everyone has choice to use browser of their choice, I personally feel that most of the net users are not so computer savvy and know the benefits of using other better browsers like Firefox.
Since most of the users are windows users and get IE for free, they are bound to use the browser, resulting in the statistics like the one above.

I am using Firefox since last 4 years now and find it no less than IE, in fact its the one browser which revolutionized browser concepts in a way.

Its free, fast, secure and very customizable web browser.




sources:
wikipedia
Market Share
Firefox