Other measurement companies have also reported a decline in Google's share of the U.S. search market after the Mozilla-Yahoo deal. Two weeks ago, comScore said that Google lost 1.6 percentage points in December, slipping to 65.4%, as Yahoo gained the same 1.6 points, climbing to 11.8% in 2014's final month.
comScore won't report January's figures until around the middle of this month.
Google has tried to limit the damage by desertions with messages on its home page enticing Firefox users to return. U.S. Firefox users with Yahoo as the browser's default provider have been asked if they want to reset their home page to google.com or if they would like to change their default to Google.
By StatCounter's data, Firefox accounted for 16.5% of all desktop U.S. browser usage during January, up from December's 15.6%, the largest one-month increase since April 2014. Worldwide, Firefox's usage share stood at 18.7% on the desktop.
Globally, Google's desktop search usage share actually climbed slightly in January, StatCounter's data showed, from 88% to 88.1%.
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