Users of the Firefox Web browser have been flocking to Netcraft's Web site to download the security company's new anti-phishing toolbar.

The free toolbar, released last week, was downloaded more than 60,000 times within hours of its release, according to Netcraft Internet Services Developer Paul Mutton. By comparison, the company's anti-phishing toolbar for Internet Explorer has been downloaded about 100,000 times since its release earlier this year, he says.

An increase in phishing attacks has been grabbing the attention of Internet merchants, end users and security providers. Phishing is a type of online fraud in which criminals send e-mails that entice users into visiting Web sites designed to look like those of a legitimate company such as a bank or auction provider. Users are asked to enter sensitive information such as a credit card number or passwords.

The scam currently is one of the most prevalent Internet threats, according to security researchers. And given the profit potential, online criminals are becoming more cunning in their attacks, by targeting scams at users of particular banks, or by geographical location, Mutton says.

Netcraft's anti-phishing toolbar seeks to thwart these kinds of threats by blocking access to reported phishing sites. Once the first recipients of a phishing e-mail report the URL of a fake site, the site is blocked for toolbar users.

Netcraft checks each reported site to verify that it is phony to avoid blocking legitimate sites, Mutton says.

The toolbar also displays the hosting location and a risk rating for each site visited. While the product is free for Internet users, Netcraft licenses a version to organizations such as banks to put their own brand on.

Netcraft has no plans to offer versions of the toolbar for other browsers.

"There's no other browser as popular as Firefox right now," Mutton says.

The open source browser, offered by the Mozilla Foundation, has nowhere near the market share of Internet Explorer, but has been steadily gaining users. As of Feb. 18, Internet Explorer had a market share in the U.S. of 90%, down from 93% in November, according to analytics firm WebSideStory.

Firefox, meanwhile, had grabbed 5.7% of the U.S. market as of February, up from 3% in November. Internet companies have taken note of its rising popularity. Yahoo began offering a toolbar for Firefox earlier this year, and Google has snapped up one of its key developers. But with success has come a downside: Security researchers are reporting an increase in threats aimed at the alternative browser.