As Mozilla notes in its report, about 85 percent of its royalty revenue in 2011 came from its contract with Google. While Mozilla has similar deals with other search engines, including Microsoft’s Bing, it’s clear that the majority of Firefox users still prefer to use Google. “The receivable from this search engine provider [Google] represented 77 percent and 64 percent of the December 31, 2011 and 2010 outstanding receivables, respectively.”
So it does look like if you use their google toolbar they make money from that.
The majority of our revenue comes from the search functionality in the Firefox browser through a number of major partners, including Google, Bing, Yahoo, Yandex, Amazon and eBay. This 2012 financial statement includes revenue from a mutually beneficial commercial agreement with Google, our largest search provider, that was signed in December 2011 and extends at least three years.
https://www.mozilla.org/en-US/foundation/annualreport/2012/