A security flaw in Firefox 3 (which also affects Firefox 2) has been reported to TippingPoint's Zero Day Initiative, a program that rewards hackers for finding flaws in software and rewards them for reporting them instead of exploiting them.
There aren't many details about the flaw, but TippingPoint describes it as a "Successful exploitation of the vulnerability could allow an attacker to execute arbitrary code. Not unlike most browser based vulnerabilities that we see these days, user interaction is required such as clicking on a link in email or visiting a malicious web page."
Mozilla, the makers of Firefox, have already been informed and are presumably at work on a fix.
Just remember, the person who reported it might not be the only one who found the security flaw. As such, be careful about clicking links on the internet. Except for this one, which just takes you to the story.
TechRadar: Firefox 3 security compromised