The warning, from the US Computer Emergency Readiness Team, will come as a serious blow to Mozilla, the non-profit body behind Firefox.

Mozilla has marketed Firefox heavily as a safer alternative to Micrsosoft’s Internet Explorer, the browser which dominates the market.

However, Secunia, the internet security firm, warned that malicious hackers could exploit a range of security holes in Firefox to gain control of computer systems and to steal personal information from computers running older versions of the software.

Mozilla has released a new version of the software - Firefox 1.5.0.2 – which can be downloaded for free from www.getfirefox.com.

"We strongly recommend that all users upgrade to this latest release," it said.

Firefox, an "opensource" application, has won a loyal army of fans, partly through a backlash against Microsoft’s dominance of the software market. Mozilla relies on an army of volunteers to develop and test the product.

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