Mozilla Foundation chairman Mitchell Baker disclosed how much money Mozilla made from their Firefox web browser in 2005: $52.9 million.
2006 wasn't listed but 2003 revenue was $2.4 million and 2004 revenue was $5.8 million. A large part of the revenue comes from the Google search in FireFox and the FF Google start page.

Now that's a nice little open source project..

FireFox: $52 Million in 2005