Jason Calacanis recently said that Mozilla made $72M just from the Google Search button that you see on the top right hand corner of every Firefox browser.
Here's how Mozilla makes money. When you search the web with that box, Mozilla gets a cut whenever you click on a sponsored link (Google Ads) displayed in search results.
Christopher Blizzard of Mozilla Corporation confirmed the rumour - the dollar amount [$72m] is not correct, though not off by an order of magnitude.
And unlike the non-profit Mozilla Foundation, the Mozilla Corporation is a taxable entity. So even the US Government is making money from Firefox.
Source: Mozilla Corp. Wiki | CRN