In a rare affront to Google’s dominance of the search market, Mozilla has announced a five-year partnership with Yahoo to make the search engine the default search experience for the Firefox browser in the United States on mobile and desktop devices. Firefox had been using Google as the browser’s default search engine since 2004, but that agreement is set to expire this month.

The agreement also provides a framework for exploring future product integrations and distribution opportunities to other markets. The deal represents the most significant partnership for Yahoo in five years. As part of this partnership, Yahoo will introduce an enhanced search experience for U.S. Firefox users that is scheduled to launch in December 2014. It features a new design created in collaboration with the Mozilla team.

Refocusing on Search

“Mozilla is an inspirational industry leader who puts users first and focuses on building forward-leaning, compelling experiences. We’re so proud that they’ve chosen us as their long-term partner in search, and I can’t wait to see what innovations we build together,” said Marissa Mayer, Yahoo CEO.

Mayer, who joined Google as employee number 20 in 1999, took the helm at Yahoo in 2012. In the last two years, she has raised the company’s profile with a string of agreements and acquisitions, including the purchase of micro-blogging site Tumblr for $1.1 billion in 2013 and nabbing NBC’s “Community” as the company’s first original television programming.

Mayer framed her latest deal as a return to the company’s roots in search. “This partnership helps to expand our reach in search and also gives us an opportunity to work closely with Mozilla to find ways to innovate more broadly in search, communications, and digital content,” she added.

“Search is a core part of the online experience for everyone, with Firefox users alone searching the Web more than 100 billion times per year globally,” said Chris Beard, Mozilla CEO. “We are excited to partner with Yahoo to bring a new, re-imagined Yahoo search experience to Firefox users in the U.S., featuring the best of the Web, and to explore new innovative search and content experiences together.”

Former Giants

Firefox has seen its popularity as a browser slide considerably in recent years with the emergence of Google Chrome as a competitor in 2008. Once the second-most popular browser after Microsoft’s Internet Explorer (IE), Firefox now ranks behind both IE and Chrome for desktop machines, and sits in fourth place in total searches (including mobile), behind Apple’s Safari.

Meanwhile, the Mozilla Foundation, the non-profit organization responsible for the development of the Firefox browser and other open source software such as the Thunderbird e-mail client, has become almost entirely dependent on Google, with the company representing 90 percent of the foundation’s revenues.

Yahoo, meanwhile, was once the most popular search engine on the Internet in the days before Google. But the company, once poster boy of the dot-com world with television commercials touting the question “Do you Yahoo?,” has been using Microsoft’s Bing search algorithm to generate its search results.