When I first began using Mozilla Firefox, one of the most useful features I found was the small search bar it included. It wasn't nearly as offensive or intrusive as just about every other search bar I'd seen before, was configurable with different engines and fit right in the browser as a whole.
Defaulting it to Google, however, was perhaps one of the best "financial" decisions that the Mozilla developers made. As it stands, for 2006 the company brought in a massive $66.8 million - the majority of which they attribute to that very search bar. While there were other sources of income, such as donations and other forms of support, the bread and butter for them does indeed seem to be search referrals:
"As in 2005 the vast majority of this revenue is associated with the search functionality in Mozilla Firefox, and the majority of that is from Google. The Firefox user base and search revenue have both increased from 2005. Search revenue increased at a lesser rate than Firefox usage growth as the rate of payment declines with volume. Other revenue sources were the Mozilla Store, public support and interest and other income on our assets."
The article brings up a very good point that is an obvious lead-in to this fact. With the increasing popularity of Firefox, the obvious reliance of Mozilla on Google for funding and Google's willingness to "acquire" other companies to add into their fold, there is clear potential for a merger. Might Google in the future make a move to buy out Mozilla, or at least buy out the Firefox browser?
That, I think, is the most interesting question of all related to Mozilla. Google has taken many other projects over, including both non-profit and for-profit companies. Google Firefox doesn't sound all that implausible now.