According to the Web metrics provider, Microsoft 's browser market share has declined by more than six percent since February 2008. The browser held a 68.15 percent market share in December -- its lowest in years.

By contrast, Mozilla Firefox's market share rose one-half of a percentage point from November and has climbed more than four percent since February to reach 21.34 percent in December.

"Reaching 20 percent worldwide market share is a significant milestone for Firefox and Mozilla," said Mozilla CEO John Lilly last month. "It's a huge achievement by the global Mozilla community, one that just a few years ago most would have considered impossible."

Mac Users Take Safari

Apple's Safari browser has also been chipping away at Internet Explorer's market share for months. Since February, Safari's market share has grown more than two percent to a 7.93 percent share in December.

However, Safari's rising market share is almost entirely due to the rising sales of Mac computers, which ship with Safari. According to Net Applications, the Mac operating system's share of the computer market has risen 2.17 percent since February.

Even Google's Chrome browser, which debuted in early September, exceeded one percent for the first time in December. Among the world's top five Web browsers, only Opera appears to be going nowhere. Opera's market share has hovered around the 0.7 percent mark since February.

Still, Net Applications warned that its reports for November and December should be eyed with caution.

"The December holiday season strongly favored residential over business usage," said the Aliso Viejo, Calif., firm, which provides applications for Webmasters and e-marketers. "This, in turn, increases the relative usage share of Mac, Firefox, Safari and other products that have relatively high residential usage."

Skewed Data

The Web metrics firm, which compiles data from approximately 160 million visitors per month, also cited three potential reasons why its November data for Firefox in particular was skewed from the norm. "All three reasons increased Firefox share, since Firefox has a much larger residential share than corporate share," Net Applications said.

For example, Firefox usage increased significantly in the days surrounding the presidential election on Nov. 4, the firm noted, then held roughly steady until the Thanksgiving holiday, when Firefox's market share suddenly jumped. The presumption is that Firefox initially got a boost when consumers flocked to the Web to follow the U.S. election results. The Black Friday sales after Thanksgiving also drew consumers to the Web at a time when most business Web activity had ground to a halt.

Additionally, the month of November was atypical in that it contained more than its share of weekend days, giving Firefox a further boost in consumer Web activity.

"The average 30-day month has 8.57 weekend days," Net Applications noted. "November had 10."