Firefox 3 has experienced rapid market share gains since its release in mid-June, helping to push the Mozilla browser's overall share to more than 19%, a Web metrics firm said Wednesday.

Since its release June 17, Firefox 3's worldwide usage share has soared to more than 4%, Net Applications reported. In the first hour of its release, the latest version of the open source browser grabbed 1% of the global market.

Firefox 3's gains came mostly from users upgrading from Firefox 2, the firm said. However, a small portion of the gains, 0.4%, came at the expense of Microsoft's Internet Explorer.

IE's market share fell to 73.01% in May to 73.75% at the end of June, Net Applications' numbers showed. Firefox overall increased its share during the same timeframe to 19.03% from 18.41%.

Since August 2007, IE's share has declined from 79%, while Firefox has risen over the last 10 months from 14.65%, according to Net Applications. During the same timeframe, Apple's Safari browser has increased its share to 6.31% from 4.71%. Much of that increase in usage is apparently due to soaring sales of Apple Mac computers, which ship with the browser.

In an InformationWeek review, Firefox 3 was found to be much faster than the previous version, primarily due to fixed memory leaks and bugs. In addition, developers had added timesaving features to streamline usage, and paid particular attention to tools that make it easier to identify favorite Web sites and return to them.

The new version, however, isn't perfect. New security features were found to be unreliable and likely to lull naive users into a false sense of security. While moderately expert users were unlikely to have a problem, from a security standpoint, InformationWeek found Firefox 3 a step backward.

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