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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