On the back of the release last week of Firefox 3.0, the Mozilla open-source browser has gained market share at the expense of rivals Internet Explorer (IE) and Safari, Net Applications has said.

Firefox's share ended the week at 19.17 percent, said Vince Vizzaccaro, the web metrics firm's executive vice president of marketing. That's up 0.76 percent from the 18.41 percent it posted for May.

Most of that gain came at the expense of Microsoft's IE, but Apple's Safari also dipped last week. IE was down 0.8 percent from May's final number, to 72.95 percent, said Vizzaccaro, while Safari was off 0.1 percent, to 6.15 percent. Opera Software, which recently unveiled version 9.5 of its flagship browser, was up slightly to 0.75 percent from May's 0.71 percent.

But Vizzaccaro warned not to read too much into Firefox's early numbers. "We do show Firefox going up, but I want to caution that it may be artificially high."

The way Net Applications tallies unique visitors, it's possible some people may have been double counted last week. A user who visited a monitored site with Firefox 2.0, for example, then downloaded and installed Firefox 3.0 and revisited the same site that day, would be counted twice, Vizzaccaro said.

It might take several weeks for a more accurate picture of Firefox's gains, if any, to be revealed, he said.

Net Applications derives its browser share data from traffic to approximately 40,000 business websites the company monitors for clients.

Mozilla, which had promoted the June 17 launch of Firefox 3.0 with a run at a one-day download record, claimed that users had grabbed more then 8.3 million copies of the browser in its first 24 hours of availability. As of 1 p.m. EDT on Monday, Mozilla's counter showed 17.9 million downloads of the new browser.

Separately, Net Applications has been tracking hour-by-hour movement of Firefox 3.0's market share. According to those figures, Firefox 3.0 had a 4 percent share by the end of Sunday. But with less than a quarter of that attributable to share stolen from rival browsers, it's clear that the bulk of 3.0's increase has come from existing users updating from earlier versions, said Vizzaccaro.

Based on trends, Vizzaccaro earlier this month projected that Firefox would reach the 20 percent share mark sometime in July.