In January, Firefox accounted for 13.7 percent of the , Aliso Viejo, Ca.-based Net Applications said, down slightly from 14 percent the month before. The dip was the first since May 2006. Since then, Firefox's share has risen continually month to month.

"It appears to be a real dip," not a statistical anomaly, said Vincent Vizzaccaro, Net Applications' executive vice president of marketing and strategic relationships. "It's still bigger than November, though, as if Firefox had a little spike in December.

"Firefox has had minor setbacks like this before," said Vizzaccaro.

More conspicuous than Firefox's slip, however, has been Safari's steady march. The Apple browser, which is based in part on the open-source Konqueror, boosted its share to 4.7 percent in January from 4.2 percent in December. A year ago, Safari held 3.1 percent of the browser market.

"The more interesting trend is on the Safari side," said Vizzaccaro. "It looks like it's taking share away from browsers in the Windows environment."