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.
The latest version of Firefox was launched in December with Yahoo replacing Google as the default search engine.
Separately, a letter from Yahoo investor Starboard Value LP to Yahoo Chief Executive
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
He also asked Mayer to tell investors her plans for Yahoo's shares in the e-commerce giant
"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.