Mozilla's Firefox web browser has gained market share at the expense of rivals Internet Explorer (IE) and Safari following the release of Firefox 3.0.

Net Applications said Firefox ended last week with 19.17 percent of the market - 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, while Safari was off 0.1 percent to 6.15 percent. Opera, which recently unveiled version 9.5 of its flagship browser, was up slightly to 0.75 percent from May's 0.71 percent.

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But Vince Vizzaccaro, Net Applications' executive vice president of marketing, 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 than 8.3 million copies of the browser in its first 24 hours of availability.