SAN FRANCISCO — Yahoo will supplant Google’s search engine on Firefox’s Web browser in the U.S., signaling Yahoo’s resolve to regain some of the ground it has lost in the most lucrative part of the Internet’s ad market.

The five-year alliance announced Wednesday will end a decade-old partnership in the U.S. between Google and the Mozilla Foundation, which oversees the Firefox browser. The tensions between Google and Mozilla had been rising since Google’s introduction of the Chrome browser in 2008 began to undercut Firefox. Google’s current contract with Mozilla expires at the end of this month, opening an opportunity for Yahoo.

Even though Chrome is now more widely used, Firefox still has a loyal audience that makes more than 100 billion worldwide search requests annually.

Yahoo is hoping to impress Firefox users as the Sunnyvale, Calif., company sets out to prove it’s still adept at Internet search after leaning on Microsoft’s technology for most of the results on Yahoo’s own website for the past four years.

Financial details of Yahoo’s Firefox contract weren’t disclosed. In a blog post, Mozilla CEO Chris Beard said the new deal offers “strong, improved economic terms” while allowing Mozilla “to innovate and advance our mission in ways that best serve our users and the Web.”

Google accounted for 90 percent, or about $274 million, of Mozilla’s royalty revenue in 2012. Mozilla hasn’t released its annual report for last year.

Besides dropping Google in the U.S., Mozilla is also shifting Firefox to Baidu’s search engine in China and Yandex in Russia. Firefox users still have the option to pull down a tab to pick Google and other search engines as their preferred way for looking up information online.