BERLIN — For the first time in a decade, Microsoft’s Internet Explorer is no longer the leading Web browser in Europe, ceding the top position to Mozilla’s Firefox, an Irish research firm that tracks Web-surfing activity said Tuesday.
While three other research firms also active in the field disputed the finding, StatCounter, a company in Dublin, said Firefox surpassed Internet Explorer as the top European browser in December, with a 38.1 percent share, compared with Explorer’s 37.5 percent.
“This is a watershed event for Europe,” said Aodhan Cullen, StatCounter’s chief executive and founder. Mr. Cullen said Microsoft had lost market share mainly to Google, whose Chrome browser share rose to 14.6 percent from 5.1 percent a year earlier.
Internet Explorer is still the clear leader worldwide, with a 46.9 percent share in December, versus 30.8 percent for Firefox and 14.9 percent for Google, StatCounter said.
Barbara Hueppe, a Mozilla spokeswoman in Berlin, said that her company tended to more closely follow other studies, which still placed Microsoft ahead of Firefox in Europe. But she said the StatCounter ranking showed the market was tightening.
“This is a very positive for us,” Ms. Hueppe said. “We have been the top browser in Germany, the biggest European market, since 2010. That has helped us a lot. The open-source nature of our software is also especially attractive to Europeans.”
Jesse Verstraete, a Microsoft spokesman in Brussels, could not immediately be reached for comment. Three other Web analytics companies, AT Internet in Bordeaux, Adtech in Dreieich, Germany, and Net Applications in California, continued to rank Internet Explorer ahead of Firefox in Europe.
According to Net Applications, Internet Explorer maintained a large lead over Firefox in December in Europe, with a 45.4 percent market share versus 33.4 percent.
Measuring Web activity is an inexact science and research companies use varying sample sizes for their rankings, which researchers said could explain the discrepancy in results.
StatCounter compiles its monthly market-share figures from three million Web sites which tell the firm which browsers were used. Mr. Cullen said the rankings are compiled based on roughly 15 billion page views to those sites each month.
Microsoft has been distributing a computer ballot screen to users in the European Union that asked them to choose a default browser. Microsoft agreed to the balloting to avoid sanctions in an antitrust case. The balloting, most researchers have concluded, has had little effect on the overall market share distribution.