Internet Explorer or Firefox? Depends on the context.
Independent (home) user, Firefox. (If only for its lack of native ActiveX support.)
Corporate environment, Internet Explorer.
Hang all the “but Firefox is faster, safer” arguments. Its a maintenance issue. An environment which standardizes upon Firefox will encounter web sites which do not give the appropriate user experience without Internet Explorer. Internet Explorer could be installed on an exception basis. Now there are two web browsers to maintain. On the face of this “reduced maintenance” overrides “faster, safer”.
Corporate environment web development. They get all expected web browsers and browser maintenance is a cost of doing business.
Corporate environment permits access to a limited, “Firefox-ready,” set of web sites. Firefox.