Yahoo! Inc. (YHOO) replaced Google Inc. (GOOGL) as the default search engine on Firefox browsers in the U.S., as Yahoo Chief Executive Officer Marissa Mayer seeks out more partnerships to boost the Web portal’s traffic and revenue. Google had been the automatic search option for the Internet browser, developed by Mountain View, California-based Mozilla Corp., since 2004. Under the agreement announced yesterday, Google, Microsoft Corp.’s Bing and other search services will be available as alternatives, No. 3-ranked Mozilla said in a blog post. Mayer, who has been working to turn around the Sunnyvale, California-based company since taking the helm two years ago, is looking for ways to bolster Yahoo’s search business, which makes up about 40 percent of sales, minus revenue passed to partner sites. Earlier this year, Yahoo, which depends on Microsoft for its search technology, struck a deal with Yelp Inc. to deliver content from the review website. “At Yahoo, we believe deeply in search -- it’s an area of investment, opportunity and growth for us,” Mayer said in a statement. “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.” Yahoo’s search service is under pressure, with the Web portal’s share of the U.S. search-advertising revenue projected to shrink to 5.6 percent in 2014 from 6.1 percent last year, according to EMarketer Inc. Google has maintained its leadership, claiming more than 70 percent of the market since 2010. An e-mail to Google seeking comment wasn’t returned. bloomberg