In return for setting Google as the default search engine on Firefox, Google pays Mozilla a substantial sum - in 2006 the total amounted to around $57 million, or 85% of the company’s total revenue. The deal was originally going to expire in 2006, but was later extended to 2008 and will now run through 2011.
http://www.techcrunch.com/2008/08/28/mozilla-extends-lucrative-deal-with-google-for-3-years/