Yahoo Inc.'s share of the search market in the United States climbed above 10% last month as the search engine became the default option on the latest version of the Firefox browser.
The news was followed Thursday by a letter from an investor expressing concern over the direction of the company on the wake of media reports that the search company is considering acquiring cable networks.
Sunnyvale, Calif.-based Yahoo claimed 10.4% of U.S. search referrals in December compared with 8.6% the month before, according to a report from StatCounter, a Web analytics company.
The December mark was Yahoo's greatest share of the market since 2009, StatCounter reported late Wednesday.
Google's share of the market fell to 75.2% last month, down from 77.3% in November and the search giant's smallest monthly share since StatCounter began tracking global search statistics in 2008.
The latest version of Firefox was launched in December with Yahoo replacing Google as the default search engine.
"The shift is not a huge amount but it is historically significant," StatCounter Chief Executive Aodhan Cullen said in an interview. "The question is, are users going to switch back to Google or are they going to stick with Yahoo?"
The day after the switch was made Yahoo's search engine was used three times as much on the latest version of Firefox as on the browser's previous version, according to StatCounter's data. Yahoo accounted for 29.4% of searches made using Firefox 34 compared with 9.64% of searches on Firefox 33, which still had Google as its default.
Separately, a letter from Yahoo investor Starboard Value LP to Yahoo Chief Executive Marissa Mayer urged the company to focus on its core business, reduce costs and consider a merger with AOL Inc.
In the letter, Starboard Chief Executive Jeffrey Smith said he was "increasingly concerned" over media reports that Yahoo is considering acquiring cable networks such as CNN and Scripps Networks Interactive, which owns several lifestyle channels including the Food Network and HGTV.
"Should you instead choose to proceed down a different path by pursuing large acquisitions and/or a cash-rich split, both of which have been speculated, such actions would be a clear indication to us that significant leadership change is required at Yahoo," he said in the letter.
A Yahoo spokeswoman said the company was not commenting on the letter.
Yahoo shares closed up $1.64, or 3.4%, at $50.23 on Thursday.