IE slipped to a 69.8% share, down from October's 71.3% and off 7.6 percentage points in the past year.

Rival browsers from Mozilla Corp., Apple Inc. and Google Inc., meanwhile, cashed in on IE's slide and posted gains for the month, according to Net Applications' data, which is culled from visitors to the thousands of Web sites the company monitors for clients.

Mozilla's Firefox, which briefly popped above the 20% share bar during October, solidified that surge in November to end the month at 20.8%, an increase of 0.8 percentage points, the largest one-month increase since March 2007.

Apple's Safari and Google's Chrome also gained in October, with Safari accounting for 7.1% of users—up 0.6 percentage points—while Google climbed just 0.1 points.

Vince Vizzaccarro, Net Applications' executive vice president of marketing, connected IE's slide—and Firefox's and Safari's impressive increases—to a pair of factors.

"The more home users who are online, using Firefox and Safari at home rather than IE, the more those browsers' shares go up," he said. With November including the Thanksgiving holiday in the U.S. and more weekend days this year—10 such days, versus an average of 8.7 per month—users were at home more than usual.

Net Applications' data has consistently shown that Firefox and Safari usage climbs on weekends and after work hours, as users surf from home computers rather than from work machines, which typically run Microsoft's IE.

"I expect Firefox and Safari to go up in December, too," said Vizzaccarro, citing the higher-than-average number of at-home days for most people this month.

The other factor, he said, is the continued climb in Firefox's share. In the past two years, for example, Firefox's usage share has grown 48%, and although there have been times when its growth has stalled, it has posted relatively steady increases.

Safari has also consistently gained share, although not at the same pace as Firefox. Last month's increase, however, was the biggest in the past two years for Apple's browser.

Google's Chrome, which came out strong in early September but later faded, closed November with 0.83%, up a bit from October's 0.74%. Chrome remains in beta and is still available only for Windows XP and Vista.

Of the top four non-Microsoft browsers, only Opera Software ASA's flagship product dropped in share during November; it fell to 0.71% from October's 0.75%.