Mozilla's Web browser applications have been downloaded over a hundred million times for free. But that doesn't mean the spawn of AOL isn't turning a profit.

Thanks to Google, Mozilla is raking in millions of dollars of revenue, which is used to pay the employees of the recently formed Mozilla Corporation and fund project and infrastructure development.

Google, which makes its share of philanthropic and open source donations, also directly employs a few Firefox developers, including lead developer Ben Goodger.

Google isn't just paying Mozilla "millions" out of the kindness of its heart. It's more so based on the same basic principle which it pays other partners and affiliates, namely search.

The default start page for Firefox includes a Google search dialogue box. It also defaults to Google search in its engine option on the Search Bar within the browser navigational toolbar. Mozilla gets paid a publicly undisclosed amount for each Google search query made from Firefox by a user.

That Google pays content and search partners, as well as AdSense participants, is not new. What is interesting, however, is the amount that Mozilla earns from its users' Google queries.

"We are very fortunate in that the search feature in Firefox is both appreciated by our users and generates revenue in the tens of millions of dollars," Mozilla head Mitchell Baker wrote in a recent blog post.

One blogger has speculated that the figure is as high as $72 million in fact.

Mozilla Corporation board member Chris Blizzard said that the $72 million figure is not correct, "though not off by an order of magnitude."

The Mozilla Corporation uses the fund to pay its employees which currently number 40 full-time equivalents (FTE) according to Baker. Most of those FTE's reside in either Mountain View, Calif., or in and around Toronto, Canada.

Browsers have not historically really been a money-making standalone business.

Mozilla's predecessor, Netscape, was also available for free and did not have the benefit (at the time) of a paying search partnership. Microsoft's Internet Explorer is a "free" inclusion for its Windows users and is not sold as a standalone product.

It's unclear whether Mozilla has another revenue-generating option from within the browser itself.

"People sometimes ask if there are other features from which we could make money. The short answer is: We don't know," Baker wrote. "Perhaps search is the only feature that will both benefit users and generate this kind of revenue."