The security flaws relate to vulnerabilities that, used in tandem, could have allowed third parties to access a Firefox user's computer by exploiting the way that it handles software installations from certain locations.
One of the reasons that Firefox became so popular in the past year is that many people had been seeking a more secure browser than Microsoft's much-exploited Internet Explorer. However, some believe that Firefox's relative health to date could be down to its small market share. As it grows in popularity, they warn that hackers may be more inclined to search for flaws to exploit. Security firm Secunia rated the exploit – albeit only a proof of concept – as "extremely critical."
Mozilla recommends that Firefox users upgrade to Firefox 1.0.4, which contains the security updates.