Search traffic (Firefox gets money for referring people to Google, and Microsoft has their own search engine);
Commercial support (lots of companies are interested in funding the development of web browsers, for all kinds of reasons);
And ulterior motives (Microsoft uses IE to shill their software platforms, various companies want to encourage standards compliance, Microsoft and Apple don't want any major gaps in their software platform, Google somehow manages to get paid almost every time you view a web page, et cetera, et cetera).