In December, Google handled 75.2 percent of US online search referrals, down from 79.3 percent a year earlier. That score is its lowest since 2008, when StatCounter started tracking the data.
Google nevertheless remains the US leader in the search market by a wide margin, ahead of Microsoft's Bing at 12.5 percent and Yahoo at 10.4 percent its highest score since 2009.
Yahoo, whose chief Marissa Mayer has repeatedly stressed that the company remains devoted to the search market that it pioneered but which Google grew to dominate, only had 7.4 percent of the search market a year before.
StatCounter said that Yahoo's resurgence coincides with the start of its partnership with Mozilla, which made Yahoo the default service for online searches done through its Firefox Web browser in the United States from December.
"The move by Mozilla has had a definite impact on US search," said StatCounter chief executive Aodhan Cullen.
Firefox, which is open-source and free, is reported by industry trackers to be the third most used Web browser in the world, behind Google's Chrome and Microsoft's Internet Explorer.