The Mozilla Foundation, maker of the open source web browser Firefox, has released a security patch to plug two critical security flaws in the browser.
Firefox has just celebrated its 50 millionth download
The flaws were found last week by net security experts. Danish firm, Secunia, called them "extremely critical".
Mozilla has now recommended people upgrade to the latest version, Firefox 1.0.4, which is a security update.
Firefox is Microsoft Internet Explorer's (IE) main rival. IE has dominated the browser market.
But many have switched to Firefox because, so far, it has had fewer security flaws than IE and is more customisable.
Although the vulnerabilities, reported on Saturday, had been identified no cases had been reported of them being exploited.
Secunia said they were "extremely critical" because they could have let cookie and history information be used to get access to personal information or access previously visited sites.
The first flaw reported fooled the browser into thinking software was being installed by a legitimate, or safe, website.
Potentially, a hacker could have taken advantage of the security flaws to secretly launch malicious code or programs.
Firefox has had more than 50 million downloads since its formal launch in November 2004.
Since then, it has chipped away at IE's dominance, but there are signs that its take-up is slowing down, according to recent figures.
IE fell a percentage in its share of the browser market, net monitoring company WebSideStory said on Tuesday.
Microsoft's browser now has a 88.9% share, while Firefox's share has risen just over a point to 6.8%. Previously, IE dominated more than 95% of the browser market.
But Microsoft is set to release another, more secure, version of IE in the summer.
The Mozilla Foundation was set up by former browser maker Netscape in 1998. Netscape dominated the browser market in the early 1990s.