It's not a huge victory for Yahoo. The increase in search share was small, from 8.6 percent in November to 10.4 percent in December -- the month when Yahoo replaced Google as Firefox's default search option in the U.S.
Still, it's Yahoo's biggest U.S. search share since 2009, according to StatCounter, a Dublin, Ireland-based company which measures activity across a range of services like search, social media and operating systems.
If the upward trajectory continues for Yahoo, it might help the company grow its ad revenue and better compete against rivals like Google and Facebook. That is, if people don't simply switch back to Google within Firefox. The native search bar in Firefox now defaults to provide results by Yahoo. But making the switch back to Google -- or Bing or Wikipedia or DuckDuckGo -- requires only a quick switch in the Firefox preferences tab.
Still, "The move by Mozilla has had a definite impact on U.S. search," said StatCounter CEO Aodhan Cullen, in the company's announcement. That's despite the fact that usage of Firefox is now eclipsed by other browsers like Google's Chrome and Microsoft's Internet Explorer, according to separate figures from Net Applications.
Google is still far and away the leader in search, with U.S. search share of 75.2 percent as of December, according to StatCounter. That's down slightly from 77.3 percent in November, making it Google's lowest U.S. search share yet since StatCounter started measuring this market in mid-2008. Microsoft's Bing came in a distant second with a share of 12.5 percent.
Google, Yahoo and Mozilla declined to comment on StatCounter's rankings.
For its search engine stats, StatCounter looks at a network of over 3 million websites and tallies every page view that comes from a search engine. The company is not tallying search queries, but rather page referrals from search engines to those 3 million sites.
Its numbers are different from ComScore's, which is widely known for tracking activity on search engines and other sites like eBay and Facebook. ComScore's most recent rankings, which only cover the month of November, prior to the Firefox switch, gave Yahoo a 10.2 percent U.S. search share, which is higher than the 8.6 percent StatCounter gave it that month.
Google's U.S. search share for November was 67 percent according to ComScore, considerably lower than the 77.3 percent cited by StatCounter.