Firefox, the internet browser that has won a cult following allowing it to chip away at Microsoft's market dominance, received a setback this week as several serious security weaknesses emerged.

News of the security problems will be especially disappointing for the Mozilla Foundation, the non-profit organisation behind the opensource browser, as it coincides with a report from www.boingboing.net which said that the popular blog had received more visitors using Firefox than Microsoft's Internet Explorer (IE) for the first time.

"This is probably the first time in Boing Boing's history, that Internet Explorer is not No1," Boing Boing wrote. "Microsoft, watch out!"

Firefox now claims more than 44 million downloads worldwide and accounts for around 5 per cent of the global market. Analysts believe that many Firefox users have defected from IE and Mozilla claims continuing strong rates of expansion - though the Microsoft product still dominates, claiming nine out of ten internet users.

Though Mozilla admits there is no "magic bullet" to ensure safety online, much of the browsers' popularity stems from its perception as a more secure browser than IE.

Mozilla itself has highlighted advice it claims came from the United States Department of Homeland Security's Computer Emergency Readiness Team "to recommend that consumers stop using Internet Explorer and switch to other browsers".

But a string of warnings posted on Mozilla's own site have prompted a backlash among some fans.

"The versions of Firefox up to version 1.0.3 have had terrible security risks," one user wrote on the Spread Firefox website. "I think these security risks have undermined the promise of Firefox as a more secure browser."

Experts have suggested that Firefox is now attracting as much attention as IE and that similar amounts of "malware" - or malicious software - are being developed for each product.

The security flaws in Firefox were issued with software "patches" by Mozilla to prevent attackers installing dangerous programs on users' computers or from acquiring personal data.

Some of the weaknesses relate to the anti-popup feature found on Firefox. This prevents adverts and other boxes appearing against a user's will. However, when users choose to allow certain pop-ups the program that powers them is allowed to run at a "privileged" level, which malicious hackers could exploit.

Mozilla is encouraging all Firefox users to update their browsers as soon as possible to plug the holes. Updates can be accessed at www.mozilla.org.

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