Mozilla Corp.’s Firefox topped Microsoft Corp.’s Internet Explorer in Europe for the first time last month to become the region’s most-used web browser, according to StatCounter, a market-research firm.

In December, Firefox’s share of the European market was 38.1 percent, with Internet Explorer at 37.5 percent and Google Inc.’s Chrome at 14.6 percent, according to StatCounter, which is based in Dublin and Boston.

It’s the first time Internet Explorer has been “dethroned from the number-one spot in a major territory,” Aodhan Cullen, StatCounter’s chief executive officer, said in a statement. “This appears to be happening because Google’s Chrome is stealing share from Internet Explorer.” Firefox is maintaining its existing share, he said.

Internet Explorer still leads in North America, with a 48.9 percent market share, against Firefox’s 26.7 percent and Chrome’s 12.8 percent, StatCounter said, adding that it samples more than 15 billion page views on 3 million websites.

Cullen said gains by Firefox and Chrome in Europe may have been prompted by an agreement between European Union regulators and Microsoft that compels the U.S. company to offer European users a choice of browsers.